It's quite possible that Firefox is preventing your computer from shutting down and prompted you to end now. I recall having that issue with a message saying that Firefox is already running. I don't recall how it went away, but it did for me.The only issue I have now is when the computer is shutting down there seems to be a program still running and I am prompted to 'End Now'
The other thing, (possibly related?), is when I shut a firefox browser window and decide to open it once again I get the message 'Firefox is already running'. Th only way I can stop this problem is to open Windows Task Manager and stop firefox.
Cheers,
SweetTech.