How to shut down Firefox when it won't open

Use the "Processes" tab of the Windows Task Manager, select firefox.ext and use the "Del" key or better use right-click and choose "Close process tree".

On Windows 7 you can get the the Windows Task Manager with "Ctrl+Shift+Esc" skipping a couple of delaying steps the old way that started with "Ctrl+Alt+Del".

To avoid (greatly reduce) the problem close Firefox with File > Exit, or the Firefox button then Exit. Closing with the "X" in the upper right hand corner of the Window closes the Window but not necessarily the application.

When you intend to close and immediately reopen Firefox, you will find the "Restartless Restart" extension will save you a lot of time and frustration instead of having to wait for Firefox to close it will close and then restart Firefox automatically..

https://addons.mozilla.org/en-US/firefox/addon/restartless-restart/

The keyboard shortcut for the extension is "Ctrl+Alt+R" and with Firefox 4 and 5, having app-tabs, you will want to close firefox properly.

Struggled with same problem, until I discovered that by going into Task Manager, bypass the tab "Applications", and go to "Processes" instead. Once there, sort by name until you find "firefox.exe" and select "End Process" at the very bottom of that window. THIS will ensure that the program is completely closed. Afterward, you should be able to launch the program w/o problems.

Why this is happening, I don't know, but I'm awfully fed up with this and spent too much time troubleshooting than I should have needed to do. I uninstalled version 6, and "upgraded" back to version 3.6.22. (Although the Mozilla Home page says this version won't be supported for much longer.) They better get a better fix for these problems, or NO ONE will be willing to try V6.

Hope this helps!