The default theme is weird. Try ash or smoke themes - those look alright. As much as I hate “any change” this is not too bad.
The sidebar is a clear step for the ai bot integration but so far it can be just closed and forgotten. It strongly reminds me ui element in Microsoft products like teams, vs code, github or azure. Mozilla will most likely follow the same path because “user are accustomed to it”.
I think this update is quite alright except for the newly pill shaped elements… looks weird to me, honestly, especially next to the vertical line like in this image
We reside in an open-source universe. People have very strong opinions on what you are suggesting. Paying for closed-source software or even for an open-source one is frowned upon. Not saying that I agree or disagree with it.
But yeah if Mozilla were to start a subscription service or offer a browser application which can be bought or a GoFundMe campaign, like Wikipedia donations, I could see contributing to it. It would kick off a shit storm no doubt but I could contribute.
Currently in Openbox, and my picom failed to start. Didn’t notice until I realised that the roundness from Firefox menus had disappeared.
Suffice to say, I’m not sure I’ll be turning picom back on. 
Been using Chromium for a good while. I have just been testing out the new Firefox over the weekend and I’m really liking the new look and the rounded corners for the tabs. I made my layout look more like how Chromium is setup and I’m quite liking it now, so I might be almost full-time back on Firefox.