Please do the following: * [[Refresh Firefox - reset add-ons and settings]] After refreshing Firefox, try youtube again.

If you're not ready to Refresh... According to several users, YouTube stopped supporting Flash. If you use an extension like YouTube Flash Player, or YouTube Flash Video Player, you can remove it now because it is obsolete and may be interfering with your use of HTML5 video on YouTube. You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either: * Ctrl+Shift+a (Mac: Command+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 cast a critical eye over the list on the right side. Any extensions that Firefox installs on its own are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Any YouTube-related to video-downloading extensions that might try to force Flash? If in doubt, disable. Often a link will appear above at least one disabled extension to restart Firefox before the change takes effect. You can complete your work on the tab and click one of the links as the last step. Any improvement? ---- After that, if problems continue, please check this diagnostic page: https://www.youtube.com/html5 If you have red boxes for H.264, see this article: [[Fix video and audio playback problems on Firefox for Windows]].