In our current implementation, :fulscreen (:-moz-full-screen for us) only applies to the topmost element in the fullscreen element stack. The spec has recently changed to apply that selector on all of the elements in that stack. This is a breaking change to the spec [1], and we decided that this change is acceptable because this behavior seems to be more sensible, and Microsoft has shipped this behavior since IE11, and unprefixed it in Edge with the same behavior, which indicates not too many authors rely on the old spec behavior. [1] https://github.com/whatwg/fullscreen/issues/18#issuecomment-134781334

Bug 1199522 - Apply :fullscreen pseudo-class to all elements in the fullscreen element stack.

Updated https://developer.mozilla.org/en-US/docs/Web/CSS/%3Afullscreen and https://developer.mozilla.org/en-US/Firefox/Releases/43#CSS