First try restarting your computer - that's the easiest way to get everything Firefox to close.

If that doesn't work, resetting Firefox should do the trick. Instructions and details are here - Refresh Firefox - reset add-ons and settings

You may be having problems in shutting down Firefox properly for some reason.

What is the exact message you receive ?

You may need to look for and kill any Firefox or plugin container processes that are still running. See "Firefox is already running but is not responding" error - How to fix

If the problem continues a screen shot of the message may help

• see How do I create a screenshot of my problem?.