--- a/content/canvas/src/WebGLContext.cpp
+++ b/content/canvas/src/WebGLContext.cpp
@@ -1019,20 +1019,22 @@ void
 WebGLContext::ForceClearFramebufferWithDefaultValues(PRUint32 mask, const nsIntRect& viewportRect)
 {
     MakeContextCurrent();
 
     bool initializeColorBuffer = 0 != (mask & LOCAL_GL_COLOR_BUFFER_BIT);
     bool initializeDepthBuffer = 0 != (mask & LOCAL_GL_DEPTH_BUFFER_BIT);
     bool initializeStencilBuffer = 0 != (mask & LOCAL_GL_STENCIL_BUFFER_BIT);
 
+    // fun GL fact: no need to worry about the viewport here, glViewport is just setting up a coordinates transformation,
+    // it doesn't affect glClear at all
+
     // prepare GL state for clearing
     gl->fDisable(LOCAL_GL_SCISSOR_TEST);
     gl->fDisable(LOCAL_GL_DITHER);
-    gl->PushViewportRect(viewportRect);
 
     if (initializeColorBuffer) {
         gl->fColorMask(1, 1, 1, 1);
         gl->fClearColor(0.f, 0.f, 0.f, 0.f);
     }
 
     if (initializeDepthBuffer) {
         gl->fDepthMask(1);
@@ -1065,18 +1067,16 @@ WebGLContext::ForceClearFramebufferWithD
     }
 
     if (initializeStencilBuffer) {
         gl->fStencilMaskSeparate(LOCAL_GL_FRONT, mStencilWriteMaskFront);
         gl->fStencilMaskSeparate(LOCAL_GL_BACK, mStencilWriteMaskBack);
         gl->fClearStencil(mStencilClearValue);
     }
 
-    gl->PopViewportRect();
-
     if (mDitherEnabled)
         gl->fEnable(LOCAL_GL_DITHER);
     else
         gl->fDisable(LOCAL_GL_DITHER);
 
     if (mScissorTestEnabled)
         gl->fEnable(LOCAL_GL_SCISSOR_TEST);
     else