Download a new copy of the Firefox program Firefox 4.0: http://www.mozilla.com/en-US/firefox/all.h

Download a new copy of the Firefox program

Firefox 4.0: http://www.mozilla.com/en-US/firefox/all.html

Firefox 3.6: http://www.mozilla.com/en-US/firefox/all-older.html

Trash the current Firefox application to do a clean (re-)install

Install the new version that you have downloaded

Your profile data is stored elsewhere in the Firefox Profile Folder, so you won't lose your bookmarks and other personal data.