I'm interested in this as well. Yes, there's many people that don't like any change, but here... From a clean Quantum UI to new Proton tabs that honestly don't look that good, and the rounded corners seem to even be badly rendered on a 1080p screen.

It also clashes with Windows 10's artstyle.

I regret updating my browser today. Even with the bugfixes and security patches. If anyone has an installer for version 90 (which allows for this about:config setting) I'm all ears. Or any extension, patch or mod with clean results.

These forums won't let me post files or links. So I can't provide the solution myself. But I kinda managed to fix this.

Rolling back is possible and Mozilla themselves provide the rollback files. That however requires creating a new local profile, and losing all extension and about:config settings, other than toolbar and other cosmetic settings within Firefox.

It's however possible through CSS themes, to edit Firefox in such a way that it looks very closely to Quantum. In my case, I found a theme whose results are not a perfect copy of Quantum UI, but it has its own charm.

If you're hard headed into getting my solution, I prepared a zip with the required files and instructions, and maybe you could OCR or manually copy from the image. Just remember to set browser.proton.enabled to True if you disabled it.

Update: add an o after the text in the image. It was cut off for some reason.

Yeah they seemed to have disabled the ability to turn it off. It's awful and the fix isn't working now. I stuck with Firefox because it's not Chrome and now they're making it worse. I have to scroll for 20secs to get to the end of my main bookmark menu.