I've been building a website in Safari, and I've just tested it in Firefox and my fixed navigation elements are behaving as if they're position is absolute.
-webkit-transition: all 300ms ease;
-moz-transition: all 300ms ease;
-o-transition: all 300ms ease;
-ms-transition: all 300ms ease;
transition: all 300ms ease;
Through the process of elimination I was able to determine that having the following in my body was causing all the problems with fixed divs in Firefox:
-o-backface-visibility: hidden; -moz-backface-visibility: hidden; -webkit-backface-visibility: hidden; backface-visibility: hidden;
I had originally added this code to prevent flickering in certain CSS transitions throughout the site, but I guess I'll have to add it to each individual class now.