You may also try cleaning out these folders:
C:\Documents and Settings\*user*\Local Settings\History
C:\Documents and Settings\*user*\Local Settings\Temporary Internet Files
C:\Documents and Settings\*user*\My Recent Documents
C:\Documents and Settings\*user*\Temp\
On Vista or Windows 7 systems, simply replace "Documents and Settings" with "Users."
See:
Lefehacker - Speed Up Slow Firefox 3.5 Windows Start-Ups
/ 
