Dude, I have almost similar problem with Firefox and the latest version:

-> Firefox is flickering when typing any letter in Google search bar or when typing any initial letter in url link

https://support.mozilla.org/en-US/questions/1261976

I have installed the complete new Firefox, even I have changed the default destination folder on my C: drive, no add-ons, no themes and even with or without HW acceleration, but the stupid Firefox is still flickering or trembling when typing initial letter in Google search bar.

Try to disable webrender.

• gfx.webrender.force-disabled = true

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• http://kb.mozillazine.org/about:config

If this didn't help then reset the pref via the right-click context menu to the default value to reverse the change

cor-el said

Try to disable webrender.

• gfx.webrender.force-disabled = true

tried - no difference - reminds me of my PC days of drivers and configurations and endless hours probing arcane settings. Now I'm a mac guy. I just expect it to work. If it doesn't - I move on. Thanks.

Did you close and restart Firefox in case this pref is cached?

What works for one, doesn't necessarily work for another, but please see :

https://www.reddit.com/r/firefox/comments/a714wr/youtube_player_flickering/

And here are some other "ideas":

https://news.softpedia.com/news/how-to-fix-the-youtube-screen-flickering-in-mozilla-firefox-525866.shtml

You can first try to set this pref to false on the about:config page.

• gfx.webrender.picture-caching = false

If it doesn't help then disable WebRender.

Bug 1559284 - WebRender picture caching implementation results in extremely choppy YouTube videos unless the mouse cursor is actively over the viewport.