In your case the problem is probably the parent.lock which indicates the profile is in use and protects the profile from being used at the time by another firefox.edxe

The following may not completely eliminate having to terminate Firefox through the Windows Task Manager but it will come very close.

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

• Firefox hangs | Troubleshooting | Firefox Support
  http://support.mozilla.com/en-US/kb/Firefox%20hangs#w_hang-at-exit

Use the Windows Task Manger to remove all running firefox.exe in the "Processes" tab of the Windows Task Manager, then restart Firefox.

very infrequent -- 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
http://kb.mozillazine.org/Profile_in_use#Remove_the_profile_lock_file

Avoiding Problems with close/restart choose either extension

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.

• "Restartless Restart" extension for Firefox 4.0+ only (2 KB download ) --
  uses keyboard shortcut "Ctrl+Alt+R" or a file menu option.
  Not for your system until you upgrade to Firefox 7 or Firefox 8
  https://addons.mozilla.org/firefox/addon/249342/
• "QuickRestart" extension (34 KB download) (older extension for those below 4.0)
  uses keyboard shortcut "Ctrl+Alt+R" or a file menu option.
  https://addons.mozilla.org/firefox/addon/3559/

Also see When trying to start Firefox I keep getting message saying "A copy of Firefox is already open. Only one copy of Firefox can be open at a time." I've started a new Profile but Profile Manager keeps telling me the Default one is still in use. | Firefox Support Forum | Firefox Help

https://support.mozilla.com/questions/666513

Supplying the exact message you get helps you get workable solutions.

I checked via task manager and closed existing version of firefox, restarted, tried to download a new version, got same message " your computer must be restarted to complete a previous upgrade of firefox. Do you want to reboot now?" Tried rebooting several times and no luck. Tried to uninstall Firefox via control panel and it said to finish installing a previous upgrade before uninstalling. It is in a loop!

Do a clean reinstall.

• Download a fresh Firefox copy and save the file to the desktop.
• Firefox 7.0.x: http://www.mozilla.com/en-US/firefox/all.html

• Uninstall your current Firefox version.
• Do not remove personal data when you uninstall the current version or you lose your bookmarks and other data in the profile folder.

• Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer.
• It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.

Your bookmarks and other profile data are stored elsewhere in the Firefox Profile Folder and won't be affected by a reinstall, but make sure that you do not select to remove personal data if you uninstall Firefox.

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Profile_backup

I cannot uninstall as it says I need to finish another install first. And I can't finish the upgrade as it keeps asking to reboot and finish installing. Task manager was used to try to stop FFox and it didn't help either.

You can delete the Firefox program folder (C:\Program Files\Mozilla Firefox\) yourself if you can't uninstall Firefox.

Thank you for the advice - it worked!