Unless you had problems with an update, one method would be starting with Firefox 4 to use "Alt" > Help > About Firefox > "Check for update" or "Apply Update". You can have updates automatically checked and wait for your approval or install them through Tools > Options > Advanced > Updates Or to instead use your download, if that is what you actually have. "Ctrl+J" to bring up downloads, start the install, then when the install starts bring down Firefox with File > Exit or the "Firefox" button > Exit, and use the Task Manager's Process tab to make sure Firefox.exe if not running -- also make sure the plugin container is also not running. then When reinstalling Firefox from a download, Firefox must be down once the installation starts (already mentioned). 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).