There are some additional solutions in our article on this topic - "Firefox is already running but is not responding" error - How to fix.

You might also want to see - Firefox hangs when you quit it as this seems to be an ongoing issue for you.

Thanks. I did look at the first thread. I know how to fix it each time, i.e., kill the process, but don't know how to prevent it from happening. As I said, something has changed. I realized after the post, that I've also recently upgraded my Norton's Internet Security to the 2013 version. That might also be related.

I don't experience Firefox hanging on close though I suppose if the process doesn't end then it is hanging on a system level. In other words, it appears to close normally every time. I'll take a look at that thread.

Thanks again for posting.