I use ungoogled-chromium with Firefox as a backup. The great thing about ungoogled-chromium is its a barebone browser, and that is exactly what I want. Only downfall is the browser does not auto update. I use change detector to get a notification when a new version is out.
Yep, it has Manifest v2 and v3 support. I believe Firefox supports Manifest v3. Manifest v2 was supposed to be removed* but Google has delayed it. From some quick searching, I was not able to find any firm date for v2 to be removed. I am with you though, when it does get removed, I will be strictly Firefox.
When v2 gets removed, why wouldn't it be possible for ungoogled-chromium to just re-add it (or not remove it in the first place)? The more I think about it, the more silly it becomes that downstream forks would not be able to re-add v2 support. I mean, it's all open source right? The code is right there. I guess it could be technically impossible, or perhaps the fundamental aspect of Chromium changes so much that you could not be able to do it.
I am thinking along the same lines. If they did remove it, they would probably try to make it a pain in the ass to add it back in, right? Certainly beyond my own abilities, however, I have faith someone would be able to do it!