The speed is a vast improvement over previous versions, but I hate that they've basically copied Chrome. Tabs on top, menus switched to mirror it, ect. Add on management sucks now as well.
Overall I like it, but I'd much rather it look and feel like previous versions with the efficiency improvements.
Also, I dunno if it's a browser issue or an issue on my end, but it seems when I'm running lots of flash dependent shit, like stickam, multiple youtube videos loading, ect I'll shoot up to 100% cpu usage. If I exit FF and use another browser the issue goes away and as soon as I open FF again it comes back til I close FF or restart.