I believe your Firefox was unable to update successfully and since not all of the files got upgraded, there's a mismatch between the old files & the new files causing that error. Do you use a security software called ZoneAlarm or ZoneAlarm Extreme Security? It may be causing the problem. If you can find the option to "Clear Virtual Data" that should allow you to reinstall or update Firefox to fix that error message. Instructions for "Clear Virtual Data" - should be in the Advanced section of ZoneAlarm's settings: http://download.zonealarm.com/bin/forcefield_x/help/wwhelp/wwhimpl/common/html/wwhelp.htm?context=ForceField&file=advancedsettings.html