A number of the menus. When I switched, I found the RSS feed dropdown mind-numbingly slow. I've since made a userChrome.css edit which I saw on Mozillazine. Even then, there's still other animations left over. I find it really distracting.

Another more minor change was, prior to this version, Firefox still allowed fixed back and forward buttons. At some point, I should probably find a fix for that as well. Animations are supposed to make the experience feel smoother, but from my point of view, it's simply more clunky.