See http://kb.mozillazine.org/Software_Update (Software Update not working properly) Remove the files in the ''updates'' and ''updates\0'' folder: XP: C:\Documents and Settings\<user>\Local Settings\Application Data\Mozilla\Firefox\Mozilla Firefox\updates %USERPROFILE%\Local Settings\Application Data\Mozilla\Firefox\Mozilla Firefox\updates C:\Program Files\Mozilla Firefox\updates ..... Vista/Windows7: C:\Users\<user>\AppData\Local\Mozilla\Firefox\Mozilla Firefox\updates %LOCALAPPDATA%\Mozilla\Firefox\Mozilla Firefox\updates C:\Users\<user>\AppData\Local\VirtualStore\Program Files\Mozilla Firefox\updates --- Do a clean reinstall and download a fresh Firefox copy from http://www.mozilla.com/firefox/all.html and save the file to the desktop. Uninstall your current Firefox version and remove the Firefox program folder before running that copy of the Firefox installer. It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling. You can skip the step to create a new profile, that is not necessary for this issue. ..... See http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall