I've found it to be a bit tricky to do a clean install of FF. There are files and profiles in LOTS of places on the computer, and the typical add/remove programs won't do it.
Check out this link, it's what Ive used on a couple different computers now to completely, cleanly, uninstall/install.
Export your bookmarks so you can save those, and if you need your saved passwords-export or take screenshots.
http://kb.mozillazine.org/Uninstalling_Firefox