This is typically caused when an add-on changes the user agent string (identifies the version of Firefox). To rectify this you can reset your user agent string. # Type '''about:config''' into the location bar and press enter # Accept the warning message that appears, you will be taken to a list of preferences # Locate the preference '''general.useragent.override''', right-click on it and select '''Reset''', this will set the user agent string back to the default in Firefox 4 There are other user agent string settings that may have been modified, but these are no longer used in Firefox 4. You can reset them if you want, for details see https://support.mozilla.com/kb/Websites+or+add-ons+incorrectly+report+incompatible+browser