Fixing issue with Firefox because of backface-visibility

This solves an issue with Firefox which disable `position: fixed;`, basically setting an element with backface-visibility disable position for any element within it.

I know, this "restore" the bug with webkit, but I do prefer that (you can still set backface-visibility on a per-element basis), because it can be fixed selectively, while the firefox issue can't be solved at all with those rules.

See this bug: https://github.com/ThrivingKings/animo.js/issues/7

Here is the firefox bug (which is 5 months old!): https://bugzilla.mozilla.org/show_bug.cgi?id=876341