Firefox process still running when windows are closed:

The only proper way to exit Firefox is to use Exit through the File menu or through the "Firefox" button, not the "X" in the upper right corner of last Firefox window.

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

Most of the problem is that you are attempting to close Firefox by closing the last window. By exiting Firefox properly you will have a much better success rate at closing Firefox. But in situations where Firefox must be down so you can safely make changes to your profile, you should still check the "Processes" tab of the Windows Task Manager to make sure Firefox is actually down.

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 .

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, (speeding up restarts)

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.
  https://addons.mozilla.org/firefox/addon/249342/

The above solution sounds like a bunch of BS. I never had to go to the file menu and select close firefox before I downloaded the latest version, version 5.0.1.

You guys a Mozilla hosed the program and ruined a good, no great thing.

FIX IT RIGHT!!!

You can make sure the underlying process is closed when you close the main window, no matter how you do that, by installing the extension MR Tech Toolkit and setting it to close child windows when the main window is closed. That might make things a little easier. Unfortunately, you will have to settle for a beta version of that extension right now. It is available at:

• http://mrte.ch/toolkit

I agree, this sucks. I never had this problem before upgrading to the newest version of Firefox. Good thing I have Chrome and Explorer to use in lieu of Firefox.