Do you have the Firefox icon on your quick launch? Close Firefox. Right click on the icon and select Open New Tab. Sometimes this works. If it opens, close it by Menu > File > Exit.

This is not a cure but will make it easier if Firefox locks up in Windows. https://support.mozilla.org/questions/997866 {web link}

If the above does not work;

Download Firefox For All languages And Systems {web link} Save the file. Then;

Start your Computer in safe mode with networking.

Starting The Computer In Safe Mode;
Free Online Encyclopedia

Using your file browser, open the Programs Folder on your computer.

Windows: C:\Program Files C:\Program Files (x86) Mac: Open the "Applications" folder. Linux: Check your user manual.

Look for, and rename any Mozilla or Firefox folders by adding .old to them.

After rebooting the computer, run a registry scanner. Then run the installer. If all goes well, remove the OLD folders when you are done.

Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox title bar.

• "3-bar" menu button > Exit (Power button)
• Windows: File > Exit
• Mac: Firefox > Quit Firefox
• Linux: File > Quit

If you use "Clear history when Firefox closes" then try to disable this feature.

• Tools > Options > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings
• https://support.mozilla.org/kb/remove-recent-browsing-search-and-download-history

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

Solved. Free Download Manager extension is causing the problem. Disable it and no more left-open Firefox processes.

That was very good work. Well Done. Please flag your last post as Solved Problem so others will know.