You can try to disable hardware acceleration in Firefox.

   Options/Preferences -> General: Performance
   remove checkmark: [ ] "Use recommended performance settings"
   remove checkmark: [ ] "Use hardware acceleration when available"
   https://support.mozilla.org/en-US/kb/performance-settings 

Close and restart Firefox after modifying the setting to make the change effective.

   https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems 

You can check if there is an update for your graphics display driver and check for hardware acceleration related issues.

   https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

Hello Dev,

Follow the step resolve the problem :

1. Clear cache and cookies
2. Disable browser add-ons
3. Disable hardware acceleration
4. Update your video driver

1.

• Click the menu button on the upper right.

• Select Library > History > Clear Recent History.

• A small window will pop up and ask you for details about what to delete. Select all check boxes and set the time range to Everything.

• Click Clear Now.

• Once everything is clear, relaunch Firefox and check if YouTube now works properly. If not, try Fix 2, below.

Fix 2: Disable browser add-ons

If you have lots of add-ons installed on your browser, they may conflict with each other and stop YouTube from working. To see if this is causing your problem, try disabling them: Click the three line button on the far right, then select Add-ons. In the Add-ons Manager tab, To disable an extension or theme, click Extensions or Themes, select the add-on you want to disable and click the Disable button next to it.

For more information please visit this link.

https://www.google.com/amp/s/www.drivereasy.com/knowledge/how-to-fix-youtube-video-freezes-audio-continues/amp/

I hope resolve your problem then reply me back.

Thank you

It definitely was not completely fixed but doing all those things seems to have reduced it greatly enough to enjoy videos at least.