When Firefox 13.0 arrived, it looked interesting. The panel of pages on a blank tab was bright and colourful in the video demo. It took a little fiddling with my settings to get those blank rectangles filled with snapshots of the pages. Then reality set in. How can I turn it off? I looked carefully at the page and at top right of the (once) blank page is a little icon of rectangles. That turned it off. I am happier with a totally blank page when I hit the new tab button. I have seen a similar approach with Safari, and I didn't like that either.
I think smooth scrolling came in earlier versions of Firefox, and I had it turned off. When Firefox 13.0 arrived, smooth scrolling was turned back on. It doesn't feel right. I prefer jerky absolute movements. The smooth scrolling would shimmy and shake and jerk up and down when I stopped scrolling. Probably St Vitus's Dance in my fingers. Probably old age. But I didn't like it, so I turned it off.
So the two big changes that I noticed in Firefox 13.0, I have turned off.