new profile, firefox will create a folder to save all the caches, sessions, extensions, themes, cookies, passwords, bookmarks, etc. that folder is called profile. check
www.mozilla.org/support/firefox/profile for its location.
you can just rename the folder "firefox" to something else (with fx closed), next time firefox starts, it will create a new one automatically. if this turns out helpful, you can just delete the old profile folder (you just renamed).