Press option+command+escape, and a dialog should come up. Do you see anything that says "firefox"? If so, force quit it. If not, go to the "activity monitor" application (in utilities folder inside applications folder) and quit anything with the word "firefox" in it.

Also, could you post the exact error message or include a screenshot? Please see How do I create a screenshot of my problem?. You can attach the saved screenshot file to your forum post by clicking the Browse... button below the Post your reply box.

Try restarting the computer. If the problem continues, post back with more information about the specific error. Creating a screenshot of the error and posting it here using these instructions might help (you could also use the Mac Grab application to create a screenshot in TIFF format, then resave it as a PNG to post here).

You could also look through this article: "Firefox is already running but is not responding" error - How to fix

Quitting something called Firefox in the Activity Monitor in the Utilities Folder did it. IT could have been a whole Firefox off on its own, but whatever it was quitting it worked !!!

Sometimes, Firefox does not close properly.

Right click the Firefox icon and select Open In New Window. This should now give you a window. Now close Firefox by;

Press the three bar button. Or type about:customizing<enter> in the address bar. Now press the Close button bottom of the right window.