diff -r 6ef20eee3f8f gfx/layers/basic/BasicCompositor.cpp
--- a/gfx/layers/basic/BasicCompositor.cpp	Thu Oct 31 09:11:56 2019 +0100
+++ b/gfx/layers/basic/BasicCompositor.cpp	Wed Dec 11 16:16:09 2019 +0100
@@ -693,9 +693,13 @@
 
   RefPtr<SourceSurface> sourceMask;
   Matrix maskTransform;
+  // Setting an alpha-mask here breaks the URL-bar on big endian (s390x)
+  // if the typed URL is too long for the textbox (automatic scrolling needed)
+#if MOZ_LITTLE_ENDIAN
   if (aTransform.Is2D()) {
     SetupMask(aEffectChain, dest, offset, sourceMask, maskTransform);
   }
+#endif
 
   CompositionOp blendMode = CompositionOp::OP_OVER;
   if (Effect* effect =
diff -r 6ef20eee3f8f gfx/layers/composite/CompositableHost.cpp
--- a/gfx/layers/composite/CompositableHost.cpp	Thu Oct 31 09:11:56 2019 +0100
+++ b/gfx/layers/composite/CompositableHost.cpp	Wed Dec 11 16:16:09 2019 +0100
@@ -91,6 +91,7 @@
   }
   MOZ_ASSERT(source);
 
+  // Alternatively: Comment out these lines where the alpha-mask is set
   RefPtr<EffectMask> effect =
       new EffectMask(source, source->GetSize(), aTransform);
   aEffects.mSecondaryEffects[EffectTypes::MASK] = effect;