I'm going to quote this, because I've had the same issue with Firefox passwords/log-in information not saving upon restart. Thank you, Gigi. :)
QUOTE(Gigi @ Apr 22 2009, 08:09 PM)

Yeah I think this is a bit of a bug with the cookies in the updated Firefox...I had this problem except every time I closed Firefox I would lose all my saved log-ins, pissed me off when I had to log in to cB every time.
This worked for me:
Start --> Run --> Type in %APPDATA% --> Mozilla --> Firefox --> Profiles
Then click on whatever folder is in there (mine is 6kvvbr26.default).
You'll see the file cookies.sqlite in there (there might also be a cookies.txt file). Delete that one (and the cookies.txt file if you have it). If you're worried about it screwing things up just cut and paste the files elsewhere.
Restart Firefox and it should be okay. HTH!