YouTube has recently changed their player from Flash to HTML5 player. This is now [https://www.youtube.com/html5 set as default]. # If this is causing your problems, you can set the player back to Flash by using the Firefox add-on [https://addons.mozilla.org/firefox/addon/youtube-flash-video-player/ YouTube Flash Video Player]. # If you are using Flash and the problem occurs, try turning of hardware acceleration as it may cause problems with playing back videos(right-click on any flash video ''> Settings > Display >'' Uncheck ''Enable Hardware Acceleration''). # Try booting in [https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Safe Mode] ans select ''Disable add-ons'' from the dialog window. If the problem doesn't occur anymore, try disabling the Firefox add-ons one by one (in normal mode) and check which one is causing trouble. # If the problems still occur, check out the following pages: * [https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems] * [https://support.mozilla.org/en-US/kb/troubleshoot-issues-with-plugins-fix-problems Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems] * [[http://kb.mozillazine.org/Flash#Troubleshooting]] * [https://support.mozilla.org/en-US/kb/keep-flash-up-to-date-and-troubleshoot-problems Flash Plugin - Keep it up to date and troubleshoot problems] I hope one of these troubleshooting steps helps.

See also: *https://support.mozilla.org/kb/fix-video-audio-problems-firefox-windows *https://support.mozilla.org/kb/fix-common-audio-and-video-issues