diff --git a/dom/canvas/ClientWebGLContext.cpp b/dom/canvas/ClientWebGLContext.cpp
--- a/dom/canvas/ClientWebGLContext.cpp
+++ b/dom/canvas/ClientWebGLContext.cpp
@@ -4253,11 +4253,12 @@
     const auto& sd = *(desc->sd);
     const auto sdType = sd.type();
     const auto& contextInfo = mNotLost->info;
 
     const auto fallbackReason = [&]() -> Maybe<std::string> {
-      auto fallbackReason = BlitPreventReason(level, offset, pi, *desc);
+      auto fallbackReason =
+          BlitPreventReason(level, offset, pi, *desc, Limits());
       if (fallbackReason) return fallbackReason;
 
       const bool canUploadViaSd = contextInfo.uploadableSdTypes[sdType];
       if (!canUploadViaSd) {
         const nsPrintfCString msg(
diff --git a/dom/canvas/TexUnpackBlob.h b/dom/canvas/TexUnpackBlob.h
--- a/dom/canvas/TexUnpackBlob.h
+++ b/dom/canvas/TexUnpackBlob.h
@@ -41,11 +41,12 @@
 struct PackingInfo;
 struct DriverUnpackInfo;
 
 Maybe<std::string> BlitPreventReason(int32_t level, const ivec3& offset,
                                      const webgl::PackingInfo&,
-                                     const TexUnpackBlobDesc&);
+                                     const TexUnpackBlobDesc&,
+                                     const Limits& limits);
 
 class TexUnpackBlob {
  public:
   const TexUnpackBlobDesc& mDesc;
   bool mNeedsExactUpload = true;
diff --git a/dom/canvas/TexUnpackBlob.cpp b/dom/canvas/TexUnpackBlob.cpp
--- a/dom/canvas/TexUnpackBlob.cpp
+++ b/dom/canvas/TexUnpackBlob.cpp
@@ -658,11 +658,12 @@
   return ValidateUnpackPixels(webgl, pi, fullRows, *this);
 }
 
 Maybe<std::string> BlitPreventReason(const int32_t level, const ivec3& offset,
                                      const webgl::PackingInfo& pi,
-                                     const TexUnpackBlobDesc& desc) {
+                                     const TexUnpackBlobDesc& desc,
+                                     const Limits& limits) {
   const auto& size = desc.size;
   const auto& unpacking = desc.unpacking;
 
   const auto ret = [&]() -> const char* {
     if (size.z != 1) {
@@ -689,12 +690,16 @@
         return "UNPACK_PREMULTIPLY_ALPHA_WEBGL is not false";
       }
     }();
     if (premultReason) return premultReason;
 
-    if (pi.format != LOCAL_GL_RGBA) {
-      return "`format` is not RGBA";
+    if (pi.format != LOCAL_GL_RGBA && pi.format != LOCAL_GL_RGB) {
+      return "`format` is not RGBA or RGB";
+    }
+
+    if (pi.format == LOCAL_GL_RGB && !limits.rgbColorRenderable) {
+      return "`format` is RGB, which is not color-renderable";
     }
 
     if (pi.type != LOCAL_GL_UNSIGNED_BYTE) {
       return "`type` is not UNSIGNED_BYTE";
     }
@@ -722,12 +727,12 @@
 
   const auto& gl = webgl->GL();
 
   // -
 
-  const auto reason =
-      BlitPreventReason(level, {xOffset, yOffset, zOffset}, pi, mDesc);
+  const auto reason = BlitPreventReason(level, {xOffset, yOffset, zOffset}, pi,
+                                        mDesc, tex->mContext->Limits());
   if (reason) {
     webgl->GeneratePerfWarning(
         "Failed to hit GPU-copy fast-path."
         " (%s) Falling back to CPU upload.",
         reason->c_str());
diff --git a/dom/canvas/WebGLContextValidate.cpp b/dom/canvas/WebGLContextValidate.cpp
--- a/dom/canvas/WebGLContextValidate.cpp
+++ b/dom/canvas/WebGLContextValidate.cpp
@@ -239,10 +239,12 @@
           [WebGLExtensionID::WEBGL_compressed_texture_astc]) {
     limits.astcHdr = gl.IsExtensionSupported(
         gl::GLContext::KHR_texture_compression_astc_hdr);
   }
 
+  limits.rgbColorRenderable = webgl.gl->IsRGBColorRenderable();
+
   if (webgl.IsWebGL2() ||
       limits.supportedExtensions[WebGLExtensionID::WEBGL_draw_buffers]) {
     gl.GetUIntegerv(LOCAL_GL_MAX_DRAW_BUFFERS, &limits.maxColorDrawBuffers);
   }
 
diff --git a/dom/canvas/WebGLTypes.h b/dom/canvas/WebGLTypes.h
--- a/dom/canvas/WebGLTypes.h
+++ b/dom/canvas/WebGLTypes.h
@@ -663,10 +663,11 @@
   uint32_t maxUniformBufferBindings = 0;
   uint32_t uniformBufferOffsetAlignment = 0;
 
   // Exts
   bool astcHdr = false;
+  bool rgbColorRenderable = false;
   uint32_t maxColorDrawBuffers = 1;
   uint64_t queryCounterBitsTimeElapsed = 0;
   uint64_t queryCounterBitsTimestamp = 0;
   uint32_t maxMultiviewLayers = 0;
 };
diff --git a/gfx/gl/GLContext.h b/gfx/gl/GLContext.h
--- a/gfx/gl/GLContext.h
+++ b/gfx/gl/GLContext.h
@@ -290,10 +290,11 @@
     mTopError = GetError();
     return IsContextLost();
   }
 
   bool HasPBOState() const { return (!IsGLES() || Version() >= 300); }
+  bool IsRGBColorRenderable() { return !IsGLES() || Version() >= 300; }
 
   /**
    * If this context is double-buffered, returns TRUE.
    */
   virtual bool IsDoubleBuffered() const { return false; }