I've been using it for a couple months now. Maybe it's just me, but I really don't care too much about how Firefox looks. I actually quite dislike curved tabs, but I can live with it. What frustrates me the most is that I don't want to wait for the fancy animations to finish before doing something. If I want a menu to come up, I want it to come up.
Most of the menu reorganizations have very little affect on me since I usually use shortcuts. But waiting for some of these animations just hurts my productiveness and I've seen other people share my dismay. When changes like this hurt the people who know how to use their browser and want to simply get things done, it's saddening.
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.
Most of the menu reorganizations have very little affect on me since I usually use shortcuts. But waiting for some of these animations just hurts my productiveness and I've seen other people share my dismay. When changes like this hurt the people who know how to use their browser and want to simply get things done, it's saddening.