diff -r 000000000000 -r 6474c204b198 layout/style/full-screen-override.css
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/layout/style/full-screen-override.css	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,23 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+
+*|*:-moz-full-screen-ancestor {
+  /* Ancestors of a full-screen element should not induce stacking contexts
+     that would prevent the full-screen element from being on top. */
+  z-index: auto;
+  /* Ancestors of a full-screen element should not be partially transparent,
+     since that would apply to the full-screen element and make the page visible
+     behind it. It would also create a pseudo-stacking-context that would let content
+     draw on top of the full-screen element. */
+  opacity: 1;
+  /* Ancestors of a full-screen element should not apply SVG masking, clipping, or
+     filtering, since that would affect the full-screen element and create a pseudo-
+     stacking context. */
+  mask: none;
+  clip: auto;
+  filter: none;
+  -moz-transform: none;
+}
+