That is the downside with Testing such developmental versions as some extensions may not be updated for such or may not work properly even if forced. Ant Video Downloader https://addons.mozilla.org/en-US/firefox/addon/video-downloader-player/ Works with Firefox 3.6 - 7.* I take it you were testing 8.0 when it was still on the Aurora channel as 8.0a2 but after the code merge that was started on sept 27 the 8.0 moved to Beta channel and the 9.0 moved to Aurora. If you do not want to be a tester then you should use Firefox 7.0.1 release or the first beta of 8.0 at most.