I found: http://support.mozilla.com/en-US/questions/873776
This seems to indicate the likely solution is disabling the McAfee Site Advisor extension. However if that isn't the case, it brings up the idea that you may want to check various extensions/plugins you are running and trying to disable them one a time to determine if one of them is causing this error.
The final and probably common suggestion would be trying to of course completely uninstall and then reinstall Firefox. Remember to backup/save bookmarks and passwords; I actually recall seeing some service for Firefox that does this via 'the cloud' allowing you to sync this information across any computer using Firefox (that you sign in to of course).