Didn't solve the problem ! How should it ... the problem came up 4-5 days ago and I didn't change anything in the settings !

Here is one I couldn't play yesterday and not today in private mode: https://www.youtube.com/watch?v=Kl3KlAr5xio

You can check the Web Console (Tools -> Web Developer) for media related messages.

• https://developer.mozilla.org/en-US/docs/Tools/Web_Console

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, DuckDuckGo PE, Disconnect, Ghostery, Privacy Badger, uBlock Origin) always make sure such extensions do not block content.

• make sure your extensions and filters are updated to the latest version
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Firefox shows a purple shield instead of a gray shield at the left end of the location/address bar in case Enhanced Tracking Protection is blocking content.

• click the shield icon for more detail and possibly disable the protection

You can check the Web Console for relevant-looking messages about blocked content.

• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop
• https://support.mozilla.org/en-US/kb/smartblock-enhanced-tracking-protection
• https://support.mozilla.org/en-US/kb/trackers-and-scripts-firefox-blocks-enhanced-track

You can try these steps in case of issues with webpages:

You can reload webpage(s) and bypass the cache to refresh possibly outdated or corrupted files.

• hold down the Shift key and left-click the Reload button
• press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• press "Command + Shift + R" (Mac)

clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Settings).

"Remove the Cookies" for websites that cause problems:

• Settings -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Settings -> Privacy & Security
  Cookies and Site Data -> Clear Data -> [X] Cached Web Content -> Clear

• https://support.mozilla.org/en-US/kb/clear-cookies-and-site-data-firefox
• https://support.mozilla.org/en-US/kb/how-clear-firefox-cache

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or userChrome.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop
• https://support.mozilla.org/en-US/kb/smartblock-enhanced-tracking-protection
• https://support.mozilla.org/en-US/kb/trackers-and-scripts-firefox-blocks-enhanced-track

NON of these methods - huge as they are - can NOT explain WHY the problem came up in the first place !

I did NOT change ANY setting in the periode from I saw the problem the first time till today ehre it's invalidating the use of FireFox ... IF it's a FireFox problem.

I think it has to do with the AddBlocker !!!!!!!!

When I run into a video, that wont play AND I turn off the AddBlocker ... it' runs ... I did it this way for - I think ' 10 videos !

Who can report this back to AddBlocker to fix it ?

I've been having problems with this all week. The problem is the YouTube video resolution. Change it from auto or whatever you use (1080p for me) and it works.

I've tried that too but it only works sometimes - shutting down the AddBlocker had been working every time until now !

But it's NOT a good solution - AddBlockerPLUS should have a notice and then fix it but I'm not able to report back to them ... I think it's because my version of AddBlocker is a FREE version.