https://bugzilla.mozilla.org/show_bug.cgi?id=1504834#c10
https://bug1504834.bmoattachments.org/attachment.cgi?id=9111147
mozilla-bmo1504834-part4.patch

# HG changeset patch
# Parent  46ea866ca3acb8bb5e1709ceb799b9c94f591dec
Problem description: Tab-titles that are too long to fit into a tab get faded out.
                     On big endian this is broken and instead of fading out, the 
                     tab gets white and the font transparent, leading to an unreadable
                     tab-title
Solution: This is not a real solution, but a hack. The real solution would have been
          to byte-swap the correct buffer, but I could not find it.
          So the next best thing is to deactivate the fading-effect. Now all tab-titles
          are readable, albeit not as pretty to look at as they could be. 
Side-effects: I have not yet found an unwanted side-effect.

diff --git gfx/2d/DrawTargetSkia.cpp gfx/2d/DrawTargetSkia.cpp
index 6bbef8d..161b96f 100644
--- gfx/2d/DrawTargetSkia.cpp
+++ gfx/2d/DrawTargetSkia.cpp
@@ -1903,6 +1903,14 @@ void DrawTargetSkia::PushLayerWithBlend(bool aOpaque, Float aOpacity,
       SkCanvas::kPreserveLCDText_SaveLayerFlag |
           (aCopyBackground ? SkCanvas::kInitWithPrevious_SaveLayerFlag : 0));
 
+#if MOZ_BIG_ENDIAN()
+  // Pushing a layer where an aMask is defined produces wrong output.
+  // We _should_ endian swap the data, but I couldn't find a workable way to do so
+  // Therefore I deactivate those layers in the meantime.
+  // The result is: Tab-titles that are longer than the available space should be faded out.
+  //                The fading doesn't work, so we deactivate the fading-effect here.
+  if (!aMask)
+#endif
   mCanvas->saveLayer(saveRec);
 
   SetPermitSubpixelAA(aOpaque);