Are you looking at the "processes" tab in the Task Manager. '''Plug-in and tasks started by Firefox may continue after attempting to close Firefox''' The "X" in the upper right-hand corner closes the Window (same as Ctrl+Shift+W) but even if it is the last Firefox window, it does not necessarily close Firefox . The only '''proper way to exit Firefox''' is to use '''Exit''' through the File menu, not the "X" in the upper right corner of last Firefox window. In the Beta that would be Alt+F then X Use the '''Windows Task Manger''' to remove all running firefox.exe in the "'''Processes'''" tab of the Windows Task Manager, then restart Firefox. If Firefox will still not start, remove the parent.lock file from the profile which is created each time Firefox is started to prevent other Firefox tasks from running, see<br> http://kb.mozillazine.org/Profile_in_use#Remove_the_profile_lock_file '''Avoiding Problems with close/restart''' :Use to close and restart Firefox after enabling or disabling an extension, switching to a new theme, or modifying configuration files, then you don't have to worry about delay or have to look in the Task Manager to see if Firefox is closed yet. Use keyboard shortcut "'''Ctrl+Alt+R'''" or a file menu option. *"'''Restartless Restart'''" extension for Firefox 4.0+ only (2 KB download ) <br>https://addons.mozilla.org/firefox/addon/249342/