I posted before how to resolve the issue where Firefox forgets your logins every time you launch it.
However, I kept having to go through that procedure in order to fix things. It got SO annoying I went back to Firefox 2. Then I went back to Firefox 3 after upgrading to Windows 7, and it started happening again.
I removed Threatfire AND Spybot, with no success- THEN I found the culprit- the Firefox AdBlock extension! Since removing this extension, the problem has gone away.
I run FF3.5 with Adblock on my work XP computer, so this issue appears to only occur with AdBlock on FF3.x under Vista or 7.