Do you mean Firefox Beta / Developer Edition, which have the new Proton UI design?

The details are still in development, but I don't think there will be a built-in option to change things more than a little bit (similar to the limited modifications in the Firefox 57-88 Photon UI design). You may need to delve into the world of the optional userChrome.css file. Many people are experimenting with different rulesets to re-theme the new UI over on https://www.reddit.com/r/FirefoxCSS/.

Note that some support for the Compact theme is still present in Firefox 89/90, but showing it in the user interface (customize mode) is behind prefs.

• browser.uidensity: 0:normal|*, 1:compact, 2:touch
• browser.compactmode.show

Do you mean Firefox Beta / Developer Edition, which have the new Proton UI design? The details are still in development, but I don't think there will be a built-in option to change things more than a little bit (similar to the limited modifications in the Firefox 57-88 Photon UI design). You may need to delve into the world of the optional userChrome.css file. Many people are experimenting with different rulesets to re-theme the new UI over on https://www.reddit.com/r/FirefoxCSS/.

Nope, I have no time to learn new rulesets to get just old version of the theme (: I think there is a lot of people, how would like to see old theme, therefore could you make it possible to downgrade only theme?

Note that some support for the Compact theme is still present in Firefox 89/90, but showing it in the user interface (customize mode) is behind prefs.

• browser.uidensity: 0:normal|*, 1:compact, 2:touch
• browser.compactmode.show

Thanks, but it doesn't downgrades, just makes a little bit smaller this panel

If this latest theme is not changed back to its original - goodbye Firefox