Hmm, seems to have been withdrawn by its author for some reason: https://addons.mozilla.org/firefox/addon/download-youtube/ To see whether you even still have it installed and enabled, could you check the Add-ons page. Either: * Command+Shift+a (Windows: Ctrl+Shift+a) * "3-bar" menu button (or Tools menu) > Add-ons * type or paste '''about:addons''' in the address bar and press Enter/Return In the left column of the Add-ons page, click Extensions. Then check your extensions list on the right. Disabled extensions cluster at the bottom. If you click the More link for your downloader, does Firefox display any message indicating that it had to be disabled for some reason? If not, can you enable it? Any luck? Note that YouTube recently stopped providing Flash-compatible formats, so some extensions that depended on those would be broken. I don't know of a specific working extension.