this article maybe help to you https://support.mozilla.com/en-US/kb/Firefox%20will%20not%20start

Thank you, heavngod. Using the Windows Task Manager did the trick...altho I did have the problem again a few days later and resolved it again with Windows Task Mgr. If the problem continues I will follow your advice and check out "Firefox hangs - Hang at exit."

All you need to do is wait. While Firefox appears closed, it's not. It doing house cleaning. Cleaning out cookies and cache. How long it takes depends upon how much garbage you have collected.

If you start the Task Manager and open the Process window, you will be able to see when Firefox has finished up and closed down.

If you forced Firefox to close using the task manager, then the house keeping process seems to freeze up.

Also, as you've already found out from the above link, some programs and plugins will prevent Firefox from closing properly. I've found this less common then simply being in a hurry. But you need to also consider it.