Your actually a version of Flash behind https://get.adobe.com/flashplayer/ Direct link scans current system and browser Note: Other software is offered in the download. <Windows Only>

https://get.adobe.com/flashplayer/otherversions/ Step 1: Select Operating System Step 2: Select A Version (Firefox, Win IE . . . .) Note: Other software is offered in the download. <Windows Only>

Ok so Youtube does not use Flash any more it uses Html5 https://www.theverge.com/2015/1/27/7926001/youtube-drops-flash-for-html5-video-default since 2015

Try https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration Try No Acceleration Then https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems?redirectlocale=en-US&redirectslug=Troubleshooting+extensions+and+themes and https://support.mozilla.org/en-US/kb/html5-audio-and-video-firefox and https://support.mozilla.org/en-US/kb/fix-video-audio-problems-firefox-windows Worst case : https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings

Please let us know if this solved your issue or if need further assistance.

Thanks for the answer and tips. I upgraded Flash as explained but it still does not work. I finally tried to disable all my add-ons and re-enable them one by one, and found out that it was Ghostery which was causing the problem. This is weird as it worked perfectly before my upgrade to Sierra and also with Sierra... So I dropped a message on Ghostery's website and hope for help from their side. (but of course if this rings a bell for you, I would gladly take any insights) Gérald

Ho, Glad you discovered the answer to your issue. If Ghostery is breaking it I am sure they will think about that. So I would guess you went through Ghostery for that site and tweaked what you allow to fix the issue.

If you haven't already, please select the answer that solves the problem (YOURS). This will help other users with similar problems find the solution more easily. Thank you for contacting Mozilla Support.