I'd the same problem, was solved uninstalling "zone alarm" toolbar

I don't have "zone alarm" I have Winamp toolbar but I use it often.

In my case it was Winamp toolbal. When I have uninstal him, problem has passed away.

I don't use any add-on toolbars but still encounter this message at shut down. Have there been any other defined causes than these two?

No. Remowe toolbar resolve this problem.

I don't have zone alarm or any additional toolbars or addons. I completely uninstalled firefox including all personal settings and bookmarks, ran ccleaner and my anti malware and anti virus software, then restarted, redownloaded a fresh download of Firebox and reinstalled. It was ok for the first day I used it but now the next day I turned on my computer and it's gone back to telling me that it can't open Firefox because it's already running and then when I try to restart my computer it gives me the stupid nsAppShell error and then the only way to shut my comp down is to hit the power button.

I have the problem of Firefox not closing completely when I close it. If you look at the processes that are running you will see firefox.exe I have used Windows Task Manager to close the process and then I could restart firefox. Firefox closes itself after a bit, say a minute.

I too have the nsAppShell-EventWindow not closing problem. That is what brought me to this site looknig for help.

I am having this issue as well. I even had a hard time getting things to shut down with task manager. I sure liked FF before this new version came out. I hope there is a fix soon.