Hi Dario19.

The about:permissions settings can be found in the Firefox Page Info window. Please let me know, if you were able to solve the issue settings the Youtube page permissions to default.

Michal

Hi, Michael Stanke: Tx for your reply! I've followed instructions, go to the permissions tab, but I can't see any option related to html5 or full screen videos. Any idea?

Personally never run into similar issue before. But what I know, in the Configuration Editor for Firefox is a preference with name full-screen-api.enabled, which can disable the full screen API required for HTML5 full screen. Can you please check this preference is set to true?

Yes, it is set to true, that is also the default value.

In that case, I would bet there is a problem outside Firefox itself, maybe some ad-on is causing it. Does it occur if you run Firefox in Safe Mode? It is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

If Firefox is not running, you can start Firefox in Safe Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem.

Yeah, bingo, that solved the problem! Once in Safe Mode, the issue didn't appear, Then, as indicated, I first disabled hardware acceleration to no effect. Also, the theme is the default firefox one, so no problem there... So there has to be some of the extensions. I went first for the last one I installed before the problem as it looked a sensible choice to begin with, and there was the problem. What was the extension causing it? User Agent Overrider And it was not a malfunction of the extension, or I think so. I selected the extension to announce my system as Windows 10/Edge when it is really Ubuntu Linux with Firefox. So I selected my real system in the extension and the problem ended. I also could see that some configurations also provoked problems with videos, while others not. I hope this can help others, and thanks a lot Michael Stanke for solving the problem, very kind and competent. Thanks mate! :)

I am glad to hear that your problem has been resolved. Thank you for contacting Mozilla Support.