diff -up mozilla-release/widget/gtk/nsLookAndFeel.cpp.1144745-2 mozilla-release/widget/gtk/nsLookAndFeel.cpp
--- mozilla-release/widget/gtk/nsLookAndFeel.cpp.1144745-2	2015-05-13 10:30:08.204367309 +0200
+++ mozilla-release/widget/gtk/nsLookAndFeel.cpp	2015-05-13 10:30:08.208367321 +0200
@@ -18,6 +18,7 @@
 
 #include <fontconfig/fontconfig.h>
 #include "gfxPlatformGtk.h"
+#include "nsScreenGtk.h"
 
 #include "gtkdrawing.h"
 #include "nsStyleConsts.h"
@@ -742,12 +743,7 @@ GetSystemFontInfo(GtkWidget *aWidget,
     // Scale fonts up on HiDPI displays.
     // This would be done automatically with cairo, but we manually manage
     // the display scale for platform consistency.
-    static auto sGdkScreenGetMonitorScaleFactorPtr = (gint (*)(GdkScreen*,gint))
-        dlsym(RTLD_DEFAULT, "gdk_screen_get_monitor_scale_factor");
-    if (sGdkScreenGetMonitorScaleFactorPtr) {
-        GdkScreen *screen = gdk_screen_get_default();
-        size *= (*sGdkScreenGetMonitorScaleFactorPtr)(screen, 0);
-    }
+    size *= nsScreenGtk::GetGtkMonitorScaleFactor();
 
     // |size| is now pixels
 
diff -up mozilla-release/widget/gtk/nsNativeThemeGTK.cpp.1144745-2 mozilla-release/widget/gtk/nsNativeThemeGTK.cpp
--- mozilla-release/widget/gtk/nsNativeThemeGTK.cpp.1144745-2	2015-05-13 10:30:08.206367315 +0200
+++ mozilla-release/widget/gtk/nsNativeThemeGTK.cpp	2015-05-13 10:30:46.966488584 +0200
@@ -6,6 +6,7 @@
 #include "nsNativeThemeGTK.h"
 #include "nsThemeConstants.h"
 #include "gtkdrawing.h"
+#include "nsScreenGtk.h"
 
 #include "nsIObserverService.h"
 #include "nsIServiceManager.h"
@@ -89,23 +90,6 @@ nsNativeThemeGTK::RefreshWidgetWindow(ns
   vm->InvalidateAllViews();
 }
 
-gint
-nsNativeThemeGTK::GdkScaleFactor()
-{
-#if (MOZ_WIDGET_GTK >= 3)
-  // Since GDK 3.10
-  static auto sGdkScreenGetMonitorScaleFactorPtr = (gint (*)(GdkScreen*, gint))
-      dlsym(RTLD_DEFAULT, "gdk_screen_get_monitor_scale_factor");
-  if (sGdkScreenGetMonitorScaleFactorPtr) {
-      // FIXME: In the future, we'll want to fix this for GTK on Wayland which
-      // supports a variable scale factor per display.
-      GdkScreen *screen = gdk_screen_get_default();
-      return sGdkScreenGetMonitorScaleFactorPtr(screen, 0);
-  }
-#endif
-    return 1;
-}
-
 
 static bool IsFrameContentNodeInNamespace(nsIFrame *aFrame, uint32_t aNamespace)
 {
@@ -796,10 +780,11 @@ nsNativeThemeGTK::GetExtraSizeForWidget(
   default:
     return false;
   }
-  aExtra->top *= GdkScaleFactor();
-  aExtra->right *= GdkScaleFactor();
-  aExtra->bottom *= GdkScaleFactor();
-  aExtra->left *= GdkScaleFactor();
+  gint scale = nsScreenGtk::GetGtkMonitorScaleFactor();
+  aExtra->top *= scale;
+  aExtra->right *= scale;
+  aExtra->bottom *= scale;
+  aExtra->left *= scale;
   return true;
 }
 
@@ -827,7 +812,7 @@ nsNativeThemeGTK::DrawWidgetBackground(n
 
   gfxRect rect = presContext->AppUnitsToGfxUnits(aRect);
   gfxRect dirtyRect = presContext->AppUnitsToGfxUnits(aDirtyRect);
-  gint scaleFactor = GdkScaleFactor();
+  gint scaleFactor = nsScreenGtk::GetGtkMonitorScaleFactor();
 
   // Align to device pixels where sensible
   // to provide crisper and faster drawing.
@@ -1066,10 +1051,11 @@ nsNativeThemeGTK::GetWidgetPadding(nsDev
         aResult->left += horizontal_padding;
         aResult->right += horizontal_padding;
 
-        aResult->top *= GdkScaleFactor();
-        aResult->right *= GdkScaleFactor();
-        aResult->bottom *= GdkScaleFactor();
-        aResult->left *= GdkScaleFactor();
+        gint scale = nsScreenGtk::GetGtkMonitorScaleFactor();
+        aResult->top *= scale;
+        aResult->right *= scale;
+        aResult->bottom *= scale;
+        aResult->left *= scale;
 
         return true;
       }
@@ -1332,7 +1318,7 @@ nsNativeThemeGTK::GetMinimumWidgetSize(n
     break;
   }
 
-  *aResult = *aResult * GdkScaleFactor();
+  *aResult = *aResult * nsScreenGtk::GetGtkMonitorScaleFactor();
 
   return NS_OK;
 }
diff -up mozilla-release/widget/gtk/nsNativeThemeGTK.h.1144745-2 mozilla-release/widget/gtk/nsNativeThemeGTK.h
--- mozilla-release/widget/gtk/nsNativeThemeGTK.h.1144745-2	2015-05-13 10:30:08.206367315 +0200
+++ mozilla-release/widget/gtk/nsNativeThemeGTK.h	2015-05-13 10:30:08.209367325 +0200
@@ -81,7 +81,6 @@ private:
                                nsIntMargin* aExtra);
 
   void RefreshWidgetWindow(nsIFrame* aFrame);
-  gint GdkScaleFactor();
 
   uint8_t mDisabledWidgetTypes[32];
   uint8_t mSafeWidgetStates[1024];    // 256 widgets * 32 bits per widget
diff -up mozilla-release/widget/gtk/nsScreenGtk.cpp.1144745-2 mozilla-release/widget/gtk/nsScreenGtk.cpp
--- mozilla-release/widget/gtk/nsScreenGtk.cpp.1144745-2	2015-05-08 18:55:27.000000000 +0200
+++ mozilla-release/widget/gtk/nsScreenGtk.cpp	2015-05-13 10:30:08.209367325 +0200
@@ -63,6 +63,23 @@ nsScreenGtk :: GetAvailRect(int32_t *out
   
 } // GetAvailRect
 
+gint
+nsScreenGtk :: GetGtkMonitorScaleFactor()
+{
+#if (MOZ_WIDGET_GTK >= 3)
+  // Since GDK 3.10
+  static auto sGdkScreenGetMonitorScaleFactorPtr = (gint (*)(GdkScreen*, gint))
+      dlsym(RTLD_DEFAULT, "gdk_screen_get_monitor_scale_factor");
+  if (sGdkScreenGetMonitorScaleFactorPtr) {
+      // FIXME: In the future, we'll want to fix this for GTK on Wayland which
+      // supports a variable scale factor per display.
+      GdkScreen *screen = gdk_screen_get_default();
+      return sGdkScreenGetMonitorScaleFactorPtr(screen, 0);
+  }
+#endif
+    return 1;
+}
+
 double
 nsScreenGtk :: GetDPIScale()
 {
@@ -127,20 +144,9 @@ nsScreenGtk :: GetColorDepth(int32_t *aC
 void
 nsScreenGtk :: Init (GdkWindow *aRootWindow)
 {
-  gint width = gdk_screen_width();
-  gint height = gdk_screen_height();
-
-  // Since GDK 3.10
-  static auto sGdkScreenGetMonitorScaleFactorPtr = (gint (*)(GdkScreen*, gint))
-      dlsym(RTLD_DEFAULT, "gdk_screen_get_monitor_scale_factor");
-  if (sGdkScreenGetMonitorScaleFactorPtr) {
-      // FIXME: In the future, we'll want to fix this for GTK on Wayland which
-      // supports a variable scale factor per display.
-      GdkScreen *screen = gdk_window_get_screen(aRootWindow);
-      gint scale = sGdkScreenGetMonitorScaleFactorPtr(screen, 0);
-      width *= scale;
-      height *= scale;
-  }
+  gint scale = nsScreenGtk::GetGtkMonitorScaleFactor();
+  gint width = gdk_screen_width()*scale;
+  gint height = gdk_screen_height()*scale;
 
   // We listen for configure events on the root window to pick up
   // changes to this rect.  We could listen for "size_changed" signals
diff -up mozilla-release/widget/gtk/nsScreenGtk.h.1144745-2 mozilla-release/widget/gtk/nsScreenGtk.h
--- mozilla-release/widget/gtk/nsScreenGtk.h.1144745-2	2015-05-08 18:55:27.000000000 +0200
+++ mozilla-release/widget/gtk/nsScreenGtk.h	2015-05-13 10:30:08.209367325 +0200
@@ -43,7 +43,8 @@ public:
   void Init(XineramaScreenInfo *aScreenInfo);
 #endif /* MOZ_X11 */
 
-  static double GetDPIScale();
+  static gint    GetGtkMonitorScaleFactor();
+  static double  GetDPIScale();
 
 private:
   uint32_t mScreenNum;