UI wise the big thing for me are tab groups. They are straight crack, and the stock Chromium implementation Brave inherits probably my favourite. Apart from th

UI wise the big thing for me are tab groups. They are straight crack, and the stock Chromium implementation Brave inherits probably my favourite. Apart from that, the main thing you can't adjust via flags is Chromium's style of using separate browser profiles instead of Firefox's multiple containers within one profile/window/session. The bigger UI differences are on mobile, where again, tab groups are crack and the implementation excellent.