Does this occur when you have already been using Firefox?

This message most commonly appears after a user has closed Firefox and attempts to reopen it directly after.
This is caused because the firefox.exe process is still running. Firefox will need to save settings and other data after a close, so it will remain running for a little bit and will require the user to wait while it closes.
But please be aware that there are many other reasons why this may occur.

More details about this can be found in this help article.

See:

• "Hang at exit": http://kb.mozillazine.org/Firefox_hangs
• "Firefox hangs when you quit it": https://support.mozilla.org/kb/Firefox+hangs

Use "Firefox > Exit" (Windows: Firefox/File > Exit; Mac: Firefox > Quit Firefox; Linux: Firefox/File > Quit) to close Firefox if you are currently doing that by clicking the close X on the Firefox title bar.

fixed thanks

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