If a right-click on the player on YouTube shows a pale menu mentioning the HTML5 player, then that is using built-in features of Firefox. If it mentions the Flash player, then it is using the Flash plugin. That would be useful to know for next steps. This is some standard guidance that addresses the most common issues with the Flash Player plugin. I'm sure you've seen some of it before, but just in case: (1) If you have any recorders/downloaders that interact with Flash media make sure they are as up-to-date as possible, or disable them temporarily. (2) Disable hardware acceleration in Firefox and in Flash (A) In Firefox, un-check the box here: "3-bar" menu button (or Tools menu) > Options > Advanced > General > "Use hardware acceleration when available" That takes effect the next time you exit and start Firefox up again. (B) In Flash, right-click the media and choose Settings, then the first mini-tab and uncheck the box to use hardware acceleration. More information in this support article from Adobe: http://helpx.adobe.com/flash-player/kb/video-playback-issues.html#main_Solve_video_playback_issues (3) Disable protected mode (Windows Vista and higher) The protected mode feature of the Flash player plugin has security benefits, but seems to have compatibility issues on some systems. There is a hidden setting for this: (A) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful. (B) In the search box above the list, type or paste flash and pause while the list is filtered (C) Double-click the dom.ipc.plugins.flash.disable-protected-mode preference to switch its value from false to true. This might not take effect until all Flash has been unloaded for a few minutes, or you close Firefox.