I would never use it as I said because of all the shit they did/do. However the google only blocked extension based adblockers not those on a browser level. These would be quite hard to block without closing down the chromium source code, which would also be hard as chromium is partially LGPL. The theming options in Opera GX are more accessible than userChrome.css with Firefox and they are more powerful than simply changing themes. The sidebar of the base Firefox has been really weak in my experience. Something like Zen+ubo however would be a more comparable (and better for that use case) option