I had a problem with importing bookmarks on a work pc, so I haven't updated mine. (I'm at work.) No big issues. Here's the only ones reported.
This list covers some of the known problems with Firefox 4 which will be resolved in future versions: All Systems
Users will experience many incompatibilities with bookmarks and history if they downgrade to any Firefox 4 beta before beta 8. Users can still downgrade back to Firefox 3.6 (see bug 620723 and bug 614439)
For some users, scrolling in the main GMail window will be slower than usual (see bug 579260)
If you try to start Firefox using a locked profile, it will crash (see bug 573369)
Some users of Adobe Reader X have experienced instability when viewing PDF documents in the browser. Uninstalling and reinstalling Adobe Reader X has been determined to resolve the issue (see bug 640901)
If you customize your toolbars to include the Location Bar or Search Bar in the Menu Bar, and then toggle the state of the Menu Bar between shown and hidden, Firefox will crash (see bug 640652)