Having just done a clean install of Windows 7 Ultimate, I was bitterly disappointed to find the FF 3.5.4 was very troublesome-not so much with crashes but constant freezing. Not helped by disabling add-ons, then complete uninstall and reinstall.
All other programs including IE 8 run quickly without freezing, and all other aspects of Windows 7 function are fine.

None of the suggestions in various forums helped, so I have just installed the 3.6 beta1 version. While nowhere near as fast as on Vista (and interestingly, for the many months I used FF 3.5 with the Windows 7 Ultimate RC on this PC and a netbook), the freezing is much less problematic. I am prepared to wait for my favourite add-ons to be updated (although much missed) as FF 3.5.4 was close to unusable on this machine.
Hope that may help another frustrated FF fan.