From MozillaZine Knowledge Base
This article deals with a program called "XPCOM:EventReceiver" not shutting down properly when Windows is shut down. This problem has been solved in many different ways for many different users.
- Turning off Firefox's anti-phishing feature may stop this from occurring. Tools -> Options -> Security and uncheck "Tell me if the site I'm visiting is a suspected forgery". 
- Uninstalling Firefox, removing its installation directory and reinstalling may fix this problem. 
Having certain extensions installed can cause this problem. If you have any of these extensions, disable them to see if they fix the problem.
This problem may be caused by other applications whose relation to Firefox is not clear.
- Google Desktop
- Outpost Firewall - make sure the loopback connection is allowed
- Video drivers - try updating or rolling back your video driver
- F-Secure Internet Security 2007  (German). To resolve the problem go to the settings: "AntiVirus & AntiSpy" - "Echtzeit-Scanning" - "Systemsteuerung" and remove the checkmark at "Systemsteuerung aktivieren". F-Secure Support claims that the problem should go away with Internet Security 2007 - Second Edition. This seems not be the case, however the configuration change still works.
This problem may be a secondary effect of Firefox hanging. Check Firefox hangs for more solutions.
Related bug reports