You can force 5.0 "compatibility" with the "[https://addons.mozilla.org/en-US/firefox/addon/add-on-compatibility-reporter/ Compatibility Reporter]" extension, or with [http://kb.mozillazine.org/Extensions.checkCompatibility extensions.checkCompatibility] using about:addons. For those that really need an earlier version of Firefox * ftp://ftp.mozilla.org/pub/firefox/releases/ :or with the latest of version 3 * http://www.mozilla.com/firefox/all-older.html When reinstalling Firefox from a download, Firefox must be down once the installation starts. When the installation finishes, don't let the install start firefox for you. Instead end the install and start Firefox in your normal manner, thus preventing creating a new profile which does not have your bookmarks cookies, etc (but your old profile would still be around and would).