Who's your ISP? (Just out of curiosity)...
Actually, if IE is fine and FF slow, and FF still slow in safe mode, i see no way it could be network based etc. Did you clear your private data?
Next thing to do would be to back up your profile (with
MozBackup) then try either a new profile(google how) and/or a re-install...
If there's still no joy after all that then go down the virus root (go to the Virus Help subforum and read the instructions there).
Cheers,
Jamey
p.s. can you link this to a specific date(/action) or has it always been like this?