Oh OK, I'd seriously advise you to upgrade to the later versions ( 3.5 ) as there are security issues involved with anything lower.
From the Mozilla website:
"Firefox 3.0.x will be maintained with security and stability updates until January 2010. All users are strongly encouraged to upgrade to Firefox 3.5"
In relation to your original query did your machine crash 2 weeks ago or anything like that? it seems to me your "cookies" were deleted from your machine somehow this can happen in a crash but more likely were deleted by the upgrade. Basically "cookies" are bits of info most websites place on your machine so they can remember when you last visited, passwords etc.
Go to Tools, Options, Privacy and ensure "Accept cookies from sites" is checked, and keep until they expire, you can also check what cookies are on the machine from here. If your cookies have been deleted you will need to logon to each site you used to logon to ( having done the above ) and ensure on each site that "Remember me?" ( as in this site ) or something like it is also ticked.
Once again I personally would not do this as Cookies are also used by companies to track your movement on the web so they can see your movement , what/when you purchase items, tailor ad's etc.
I know it's a pain to enter your logon details over and over but like most things it's a trade off the decision is yours
Having said all of the above it could very simply be that the folks at Mozilla removed this ability in later editions due to security concerns.
Would like to help more but can't test it properly having the upgraded version, sorry.