First, make a backup of your computer for safekeeping. To back up Firefox, see [https://support.mozilla.com/en-US/kb/Backing+up+your+information Backing up your information]. Next, you can "kill" an invisible-but-still-running instance of Firefox using the Windows Task Manager. However, if you are not familiar with that, it might be easier just to restart Windows. Once Firefox 3.6 has truly stopped running, you should be able to install Fx4 and migrate your profile data. If your settings do not come over cleanly, you can create a new profile and use your backup to restore bookmarks and other key settings. Hopefully you won't need these articles: To create a new (blank) profile: [http://support.mozilla.com/kb/Managing+profiles Managing profiles]. To move key settings like bookmarks from your damaged profile to the new profile, see: [https://support.mozilla.com/en-US/kb/Recovering+important+data+from+an+old+profile Recovering important data from an old profile]. Hope this helps.