Before going any further
1) when you uninstall Firefox usually the user profile is not destroyed unless you specifically request it. Reinstalling the program usually finds the profile again. Did you actually uninstall or delete the profile?
2) Is you e-mail IMAP or POP? Which company is it with?
3) are you sure that spam controls are not running on the e-mail providers server? It strikes me that mail is arriving pre-flagged
4) which operating system are you running
5) what security / antivirus software do you have on this machine?
As mixture says, DON'T take this to PMs. The more people who can offer an insight into this the better