Hmm, while that's very interesting, and I'm glad that works for you, nearly all of those things sound terrible to me, particularly that last point. Still all those behaviors seem achievable in Firefox using addons if that's actually what you wanted. The most trivial one is obviously auto-closing old tabs. There's already I believe a couple extensions that do exactly that. Syncing tabs between windows would also be pretty trivial, and FF already has the option to sync open tabs and windows between devices. The tab groups thing sounds a bit like profiles in Firefox although not quite. That one would probably require the most work to replicate.

At the end of the day though, any Chromium based browser is really just Chrome with different porcelain on top. Ultimately Google is still calling the shots there and Arc better be careful hitching their wagon to them. That could go very badly for them if they ever do become a big enough thorn in Google's side.