Do you have the same issue with Firefox in Troubleshoot Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-firefox-in-4troubleshoot-modesf5safe-modesf

Are you running pro or home version of Windows? If pro, you should have sandbox enabled for testing. https://www.howtogeek.com/399290/how-to-use-windows-10s-new-sandbox-to-safely-test-apps/

The issues is gone in troubleshooting mode, I am running Windows home

Edit: After turning Off troubleshoot mode, YouTube now works fine at 1080 and uses it as the auto resolution so i guess that fixed it????

Sounds like an add-on may be the issue, but if you're a happy camper mark this issue as resolved and have a nice day.

You can also clear the clear the browser cache and startup cache. see screenshot https://support.mozilla.org/en-US/kb/how-clear-firefox-cache#

I found the solution to this to disable and renable the extension uBlock on the browser.

Have the same issue as above, only on Firefox. Tried all the troubleshooting tips as above.

How I solved the problem Backed up bookmarks From user/name/appdata/roaming/mozilla/firefox/profiles/"your profile" You can get your default profile name from profiles.ini Copy key4.db and logins.json to a temp directory Rename user/name/appdata/roaming/mozilla Rename c:/program files/mozilla firefox Install Firefox new

After the install, restore bookmarks and the key4.db and logins.json to the new profile

After starting Firefox, you need to reset your toolbars and update your settings Also, for YouTube videoauto start, you will need to

   change the settings/security/Autoplay to Allow Audio and Video

You reeally need to have some tech experience to do this.

This issue is widespread. My suspicion is that Google is intentionally slowing down YouTube on Firefox in order to promote Google Chrome.

Edit: Spelling Mistake

Note that Google might do this on YouTube if it detects a content blocking extension that blocks ads.

I had the same problem. I found old solutions to the problem. I know, but I was desperate. I did the following.

about:config media.wmf.zero-copy-nv12-texture - to false gfx.direct3d11.reuse-decoder-device - to false webgl.disabled - to true

Hardware acceleration turned off in the settings and closed Firefox. Restarted Firefox played a video on Youtube, no problems with 1440p . Closed the Youtube tab and undid the setting in about:config and turned on hardware acceleration again and restarted Firefox. So far everything is running normally.