Is it this add-on - https://addons.mozilla.org/firefox/addon/screenshot/ Prior to the release of Firefox 5 which is due out on June 21st, Mozilla will be checking add-ons that are hosted on AMO and work with Firefox 4 to see if they work with Firefox 5, and for those that do they will update the compatibility details to let the add-on install on Firefox 5. If you are happy testing new versions of Firefox prior to the release, you may want to install the Add-on Compatibility Reporter extension - https://addons.mozilla.org/firefox/addon/add-on-compatibility-reporter/ With this extension you can enable all of your current extensions to test them to see if they work with the development version of Firefox.