diff --git a/widget/gtk/gtk3drawing.c b/widget/gtk/gtk3drawing.c

--- a/widget/gtk/gtk3drawing.c

+++ b/widget/gtk/gtk3drawing.c

@@ -2098,18 +2098,23 @@ moz_gtk_progress_chunk_paint(cairo_t *cr

     return MOZ_GTK_SUCCESS;

 }

 gint

 moz_gtk_get_tab_thickness(void)

 {

     GtkBorder border;

     GtkStyleContext * style;

+    gboolean has_tab_gap;

     ensure_tab_widget();

+    gtk_widget_style_get(gTabWidget, "has-tab-gap", &has_tab_gap, NULL);

+    if (!has_tab_gap)

+      return 0; /* don't use ythickness for tabs without a gap */

+

     style = gtk_widget_get_style_context(gTabWidget);

     gtk_style_context_add_class(style, GTK_STYLE_CLASS_NOTEBOOK);

     gtk_style_context_get_border(style, 0, &border);

     if (border.top < 2)

         return 2; /* some themes don't set ythickness correctly */

     return border.top;

@@ -2140,172 +2145,196 @@ moz_gtk_tab_paint(cairo_t *cr, GdkRectan

      * When it is selected, we overwrite the adjacent border of the tabpanel

      * touching the tab with a pierced border (called "the gap") to make the

      * tab appear physically attached to the tabpanel; see details below. */

     GtkStyleContext* style;

     GdkRectangle tabRect;

     GdkRectangle focusRect;

     GdkRectangle backRect;

+    gboolean has_tab_gap;

     int initial_gap = 0;

     ensure_tab_widget();

     gtk_widget_set_direction(gTabWidget, direction);

     style = gtk_widget_get_style_context(gTabWidget);    

     gtk_style_context_save(style);

     moz_gtk_tab_prepare_style_context(style, flags);

-    tabRect = *rect;

-

-    if (flags & MOZ_GTK_TAB_FIRST) {

-        gtk_widget_style_get (gTabWidget, "initial-gap", &initial_gap, NULL);

-        tabRect.width -= initial_gap;

-

-        if (direction != GTK_TEXT_DIR_RTL) {

-            tabRect.x += initial_gap;

+    gtk_widget_style_get(gTabWidget, "has-tab-gap", &has_tab_gap, NULL);

+    if (has_tab_gap) {

+        tabRect = *rect;

+        if (flags & MOZ_GTK_TAB_FIRST) {

+            gtk_widget_style_get (gTabWidget, "initial-gap", &initial_gap, NULL);

+            tabRect.width -= initial_gap;

+

+            if (direction != GTK_TEXT_DIR_RTL) {

+                tabRect.x += initial_gap;

+            }

         }

-    }

-

-    focusRect = backRect = tabRect;

-

-    if ((flags & MOZ_GTK_TAB_SELECTED) == 0) {

-        /* Only draw the tab */

-        gtk_render_extension(style, cr,

-                             tabRect.x, tabRect.y, tabRect.width, tabRect.height,

-                            (flags & MOZ_GTK_TAB_BOTTOM) ?

-                                GTK_POS_TOP : GTK_POS_BOTTOM );

+

+        focusRect = backRect = tabRect;

+

+        if ((flags & MOZ_GTK_TAB_SELECTED) == 0) {

+            /* Only draw the tab */            

+            gtk_render_extension(style, cr,

+                                 tabRect.x, tabRect.y, tabRect.width, tabRect.height,

+                                (flags & MOZ_GTK_TAB_BOTTOM) ?

+                                    GTK_POS_TOP : GTK_POS_BOTTOM );

+        } else {

+            /* Draw the tab and the gap

+             * We want the gap to be positioned exactly on the tabpanel top

+             * border; since tabbox.css may set a negative margin so that the tab

+             * frame rect already overlaps the tabpanel frame rect, we need to take

+             * that into account when drawing. To that effect, nsNativeThemeGTK

+             * passes us this negative margin (bmargin in the graphic below) in the

+             * lowest bits of |flags|.  We use it to set gap_voffset, the distance

+             * between the top of the gap and the bottom of the tab (resp. the

+             * bottom of the gap and the top of the tab when we draw a bottom tab),

+             * while ensuring that the gap always touches the border of the tab,

+             * i.e. 0 <= gap_voffset <= gap_height, to avoid surprinsing results

+             * with big negative or positive margins.

+             * Here is a graphical explanation in the case of top tabs:

+             *             ___________________________

+             *            /                           \

+             *           |            T A B            |

+             * ----------|. . . . . . . . . . . . . . .|----- top of tabpanel

+             *           :    ^       bmargin          :  ^

+             *           :    | (-negative margin,     :  |

+             *  bottom   :    v  passed in flags)      :  |       gap_height

+             *    of  -> :.............................:  |    (the size of the

+             * the tab   .       part of the gap       .  |  tabpanel top border)

+             *           .      outside of the tab     .  v

+             * ----------------------------------------------

+             *

+             * To draw the gap, we use gtk_paint_box_gap(), see comment in

+             * moz_gtk_tabpanels_paint(). This box_gap is made 3 * gap_height tall,

+             * which should suffice to ensure that the only visible border is the

+             * pierced one.  If the tab is in the middle, we make the box_gap begin

+             * a bit to the left of the tab and end a bit to the right, adjusting

+             * the gap position so it still is under the tab, because we want the

+             * rendering of a gap in the middle of a tabpanel.  This is the role of

+             * the gints gap_{l,r}_offset. On the contrary, if the tab is the

+             * first, we align the start border of the box_gap with the start

+             * border of the tab (left if LTR, right if RTL), by setting the

+             * appropriate offset to 0.*/

+            gint gap_loffset, gap_roffset, gap_voffset, gap_height;

+

+            /* Get height needed by the gap */

+            gap_height = moz_gtk_get_tab_thickness();

+

+            /* Extract gap_voffset from the first bits of flags */

+            gap_voffset = flags & MOZ_GTK_TAB_MARGIN_MASK;

+            if (gap_voffset > gap_height)

+                gap_voffset = gap_height;

+

+            /* Set gap_{l,r}_offset to appropriate values */

+            gap_loffset = gap_roffset = 20; /* should be enough */

+            if (flags & MOZ_GTK_TAB_FIRST) {

+                if (direction == GTK_TEXT_DIR_RTL)

+                    gap_roffset = initial_gap;

+                else

+                    gap_loffset = initial_gap;

+            }

+

+            if (flags & MOZ_GTK_TAB_BOTTOM) {

+                /* Draw the tab on bottom */

+                focusRect.y += gap_voffset;

+                focusRect.height -= gap_voffset;

+

+                gtk_render_extension(style, cr,

+                                     tabRect.x, tabRect.y + gap_voffset, tabRect.width,

+                                     tabRect.height - gap_voffset, GTK_POS_TOP);

+

+                gtk_style_context_remove_region(style, GTK_STYLE_REGION_TAB);

+

+                backRect.y += (gap_voffset - gap_height);

+                backRect.height = gap_height;

+

+                /* Draw the gap; erase with background color before painting in

+                 * case theme does not */

+                gtk_render_background(style, cr, backRect.x, backRect.y,

+                                     backRect.width, backRect.height);

+                cairo_save(cr);

+                cairo_rectangle(cr, backRect.x, backRect.y, backRect.width, backRect.height);

+                cairo_clip(cr);

+

+                gtk_render_frame_gap(style, cr,

+                                     tabRect.x - gap_loffset,

+                                     tabRect.y + gap_voffset - 3 * gap_height,

+                                     tabRect.width + gap_loffset + gap_roffset,

+                                     3 * gap_height, GTK_POS_BOTTOM,

+                                     gap_loffset, gap_loffset + tabRect.width);

+                cairo_restore(cr);

+            } else {

+                /* Draw the tab on top */

+                focusRect.height -= gap_voffset;

+                gtk_render_extension(style, cr,

+                                     tabRect.x, tabRect.y, tabRect.width,

+                                     tabRect.height - gap_voffset, GTK_POS_BOTTOM);

+

+                gtk_style_context_remove_region(style, GTK_STYLE_REGION_TAB);

+

+                backRect.y += (tabRect.height - gap_voffset);

+                backRect.height = gap_height;

+

+                /* Draw the gap; erase with background color before painting in

+                 * case theme does not */

+                gtk_render_background(style, cr, backRect.x, backRect.y,

+                                      backRect.width, backRect.height);

+

+                cairo_save(cr);

+                cairo_rectangle(cr, backRect.x, backRect.y, backRect.width, backRect.height);

+                cairo_clip(cr);

+

+                gtk_render_frame_gap(style, cr,

+                                     tabRect.x - gap_loffset,

+                                     tabRect.y + tabRect.height - gap_voffset,

+                                     tabRect.width + gap_loffset + gap_roffset,

+                                     3 * gap_height, GTK_POS_TOP,

+                                     gap_loffset, gap_loffset + tabRect.width);

+                cairo_restore(cr);

+            }

+

+            if (state->focused) {

+              /* Paint the focus ring */

+              GtkBorder border;

+              gtk_style_context_get_border(style, GetStateFlagsFromGtkWidgetState(state), &border);

+

+              focusRect.x += border.left;

+              focusRect.width -= (border.left + border.right);

+              focusRect.y += border.top;

+              focusRect.height -= (border.top + border.bottom);

+

+              gtk_render_focus(style, cr,

+                              focusRect.x, focusRect.y, focusRect.width, focusRect.height);

+            }

+        }

     } else {

-        /* Draw the tab and the gap

-         * We want the gap to be positioned exactly on the tabpanel top

-         * border; since tabbox.css may set a negative margin so that the tab

-         * frame rect already overlaps the tabpanel frame rect, we need to take

-         * that into account when drawing. To that effect, nsNativeThemeGTK

-         * passes us this negative margin (bmargin in the graphic below) in the

-         * lowest bits of |flags|.  We use it to set gap_voffset, the distance

-         * between the top of the gap and the bottom of the tab (resp. the

-         * bottom of the gap and the top of the tab when we draw a bottom tab),

-         * while ensuring that the gap always touches the border of the tab,

-         * i.e. 0 <= gap_voffset <= gap_height, to avoid surprinsing results

-         * with big negative or positive margins.

-         * Here is a graphical explanation in the case of top tabs:

-         *             ___________________________

-         *            /                           \

-         *           |            T A B            |

-         * ----------|. . . . . . . . . . . . . . .|----- top of tabpanel

-         *           :    ^       bmargin          :  ^

-         *           :    | (-negative margin,     :  |

-         *  bottom   :    v  passed in flags)      :  |       gap_height

-         *    of  -> :.............................:  |    (the size of the

-         * the tab   .       part of the gap       .  |  tabpanel top border)

-         *           .      outside of the tab     .  v

-         * ----------------------------------------------

-         *

-         * To draw the gap, we use gtk_paint_box_gap(), see comment in

-         * moz_gtk_tabpanels_paint(). This box_gap is made 3 * gap_height tall,

-         * which should suffice to ensure that the only visible border is the

-         * pierced one.  If the tab is in the middle, we make the box_gap begin

-         * a bit to the left of the tab and end a bit to the right, adjusting

-         * the gap position so it still is under the tab, because we want the

-         * rendering of a gap in the middle of a tabpanel.  This is the role of

-         * the gints gap_{l,r}_offset. On the contrary, if the tab is the

-         * first, we align the start border of the box_gap with the start

-         * border of the tab (left if LTR, right if RTL), by setting the

-         * appropriate offset to 0.*/

-        gint gap_loffset, gap_roffset, gap_voffset, gap_height;

-

-        /* Get height needed by the gap */

-        gap_height = moz_gtk_get_tab_thickness();

-

-        /* Extract gap_voffset from the first bits of flags */

-        gap_voffset = flags & MOZ_GTK_TAB_MARGIN_MASK;

-        if (gap_voffset > gap_height)

-            gap_voffset = gap_height;

-

-        /* Set gap_{l,r}_offset to appropriate values */

-        gap_loffset = gap_roffset = 20; /* should be enough */

-        if (flags & MOZ_GTK_TAB_FIRST) {

-            if (direction == GTK_TEXT_DIR_RTL)

-                gap_roffset = initial_gap;

-            else

-                gap_loffset = initial_gap;

+        gtk_render_background(style, cr, rect->x, rect->y, rect->width, rect->height);

+        gtk_render_frame(style, cr, rect->x, rect->y,rect->width, rect->height);

+

+        if (state->focused) {

+          /* Paint the focus ring */

+          GtkBorder padding;

+

+          GtkStateFlags state_flags = GetStateFlagsFromGtkWidgetState(state);

+

+          focusRect = *rect;

+          gtk_style_context_get_padding(style, state_flags, &padding);

+

+          focusRect.x += padding.left;

+          focusRect.width -= (padding.left + padding.right);

+          focusRect.y += padding.top;

+          focusRect.height -= (padding.top + padding.bottom);

+

+          gtk_render_focus(style, cr,

+                          focusRect.x, focusRect.y, focusRect.width, focusRect.height);

         }

-

-        if (flags & MOZ_GTK_TAB_BOTTOM) {

-            /* Draw the tab on bottom */

-            focusRect.y += gap_voffset;

-            focusRect.height -= gap_voffset;

-

-            gtk_render_extension(style, cr,

-                                 tabRect.x, tabRect.y + gap_voffset, tabRect.width,

-                                 tabRect.height - gap_voffset, GTK_POS_TOP);

-

-            gtk_style_context_remove_region(style, GTK_STYLE_REGION_TAB);

-

-            backRect.y += (gap_voffset - gap_height);

-            backRect.height = gap_height;

-

-            /* Draw the gap; erase with background color before painting in

-             * case theme does not */

-            gtk_render_background(style, cr, backRect.x, backRect.y,

-                                 backRect.width, backRect.height);

-            cairo_save(cr);

-            cairo_rectangle(cr, backRect.x, backRect.y, backRect.width, backRect.height);

-            cairo_clip(cr);

-

-            gtk_render_frame_gap(style, cr,

-                                 tabRect.x - gap_loffset,

-                                 tabRect.y + gap_voffset - 3 * gap_height,

-                                 tabRect.width + gap_loffset + gap_roffset,

-                                 3 * gap_height, GTK_POS_BOTTOM,

-                                 gap_loffset, gap_loffset + tabRect.width);

-            cairo_restore(cr);

-        } else {

-            /* Draw the tab on top */

-            focusRect.height -= gap_voffset;

-            gtk_render_extension(style, cr,

-                                 tabRect.x, tabRect.y, tabRect.width,

-                                 tabRect.height - gap_voffset, GTK_POS_BOTTOM);

-

-            gtk_style_context_remove_region(style, GTK_STYLE_REGION_TAB);

-

-            backRect.y += (tabRect.height - gap_voffset);

-            backRect.height = gap_height;

-

-            /* Draw the gap; erase with background color before painting in

-             * case theme does not */

-            gtk_render_background(style, cr, backRect.x, backRect.y,

-                                  backRect.width, backRect.height);

-

-            cairo_save(cr);

-            cairo_rectangle(cr, backRect.x, backRect.y, backRect.width, backRect.height);

-            cairo_clip(cr);

-

-            gtk_render_frame_gap(style, cr,

-                                 tabRect.x - gap_loffset,

-                                 tabRect.y + tabRect.height - gap_voffset,

-                                 tabRect.width + gap_loffset + gap_roffset,

-                                 3 * gap_height, GTK_POS_TOP,

-                                 gap_loffset, gap_loffset + tabRect.width);

-            cairo_restore(cr);

-        }

-    }

-

-    if (state->focused) {

-      /* Paint the focus ring */

-      GtkBorder border;

-      gtk_style_context_get_border(style, GetStateFlagsFromGtkWidgetState(state), &border);

-

-      focusRect.x += border.left;

-      focusRect.width -= (border.left + border.right);

-      focusRect.y += border.top;

-      focusRect.height -= (border.top + border.bottom);

-

-      gtk_render_focus(style, cr,

-                      focusRect.x, focusRect.y, focusRect.width, focusRect.height);

     }

     gtk_style_context_restore(style);

     return MOZ_GTK_SUCCESS;

 }

 /* tab area*/

@@ -2963,48 +2992,53 @@ moz_gtk_get_widget_border(GtkThemeWidget

 }

 gint

 moz_gtk_get_tab_border(gint* left, gint* top, gint* right, gint* bottom, 

                        GtkTextDirection direction, GtkTabFlags flags)

 {

     GtkStyleContext* style;    

     int tab_curvature;

+    gboolean has_tab_gap;

     ensure_tab_widget();

     style = gtk_widget_get_style_context(gTabWidget);

     gtk_style_context_save(style);

     moz_gtk_tab_prepare_style_context(style, flags);

     // TODO add_style_border() should be replaced

     // with focus-line-width and focus-padding

     // see Bug 877605

     *left = *top = *right = *bottom = 0;

-    moz_gtk_add_style_border(style, left, top, right, bottom);

     moz_gtk_add_style_padding(style, left, top, right, bottom);

-    gtk_widget_style_get (gTabWidget, "tab-curvature", &tab_curvature, NULL);

-    *left += tab_curvature;

-    *right += tab_curvature;

-

-    if (flags & MOZ_GTK_TAB_FIRST) {

-      int initial_gap;

-      gtk_widget_style_get (gTabWidget, "initial-gap", &initial_gap, NULL);

-      if (direction == GTK_TEXT_DIR_RTL)

-      	*right += initial_gap;

-      else

-      	*left += initial_gap;

-    }

-

-    // Top tabs have no bottom border, bottom tabs have no top border

-    if (flags & MOZ_GTK_TAB_BOTTOM) {

-      *top = 0;

-    } else {

-      *bottom = 0;

+    gtk_widget_style_get(gTabWidget, "has-tab-gap", &has_tab_gap, NULL);

+    if (has_tab_gap) {

+      moz_gtk_add_style_border(style, left, top, right, bottom);

+

+      gtk_widget_style_get (gTabWidget, "tab-curvature", &tab_curvature, NULL);

+      *left += tab_curvature;

+      *right += tab_curvature;

+

+      if (flags & MOZ_GTK_TAB_FIRST) {

+        int initial_gap;

+        gtk_widget_style_get (gTabWidget, "initial-gap", &initial_gap, NULL);

+        if (direction == GTK_TEXT_DIR_RTL)

+          *right += initial_gap;

+        else

+          *left += initial_gap;

+      }

+

+      // Top tabs have no bottom border, bottom tabs have no top border

+      if (flags & MOZ_GTK_TAB_BOTTOM) {

+        *top = 0;

+      } else {

+        *bottom = 0;

+      }

     }

     gtk_style_context_restore(style);

     return MOZ_GTK_SUCCESS;

 }

 gint

diff --git a/widget/gtk/nsNativeThemeGTK.cpp b/widget/gtk/nsNativeThemeGTK.cpp

--- a/widget/gtk/nsNativeThemeGTK.cpp

+++ b/widget/gtk/nsNativeThemeGTK.cpp

@@ -756,16 +756,18 @@ nsNativeThemeGTK::GetExtraSizeForWidget(

       return true;

     }

   case NS_THEME_TAB :

     {

       if (!IsSelectedTab(aFrame))

         return false;

       gint gap_height = moz_gtk_get_tab_thickness();

+      if (!gap_height)

+        return false;

       int32_t extra = gap_height - GetTabMarginPixels(aFrame);

       if (extra <= 0)

         return false;

       if (IsBottomTab(aFrame)) {

         aExtra->top = extra;

       } else {