I'm using Firefox 3.5.4 on two Core2 Duo/XP computers and one Core i7 Win7 computer. On one of the XP machines, I have to login for every site and start page I use, once I close the browser, on the other XP machine, it stays logged in on everything as long as I use them regularly. My wife uses the other machine, and she uses IE8, rather than the FF browser I use, if I ever get on that machine... so I don't know what's happening on that one. As long as she isn't complaining, I'm happy.
On the Win7 computer, FF won't always open, but if I open Task Manager and end the firefox.exe process, I can once again open FF, as usual.
I was having problems with FF for the two weeks I ran Vista, before, I loaded my free Win7 upgrade, but I don't think it was quite this bad. Hopefully, there will be a quick SP release and these smallish problems will be settled. I'd say I could move to a Mac, but who wants to build a Mac? My Powerbook G4 is still clucking along, anyway.
As far as Firefox is concerned... I may have to switch to Opera, Safari, or something else... but my Roboform logger won't work on anything but FF and IE... I'll have to open each login, one at a time, and do a lot of copy/paste to use another browser. I wish they could solve these problems.