And while Brave might be based on Chromium, it is distinct; in addition to not crippling nativewebrequest as chrome will, it's native adblocker is compatible with the same lists as ublock origin. So I would go with Brave :)
Ironically, though, the larger Firefox's market share, the more Google will pay to be the default search engine in Firefox. Yes, it's perverse and a little gross that we depend on Google to such a large degree to keep Mozilla and Firefox funded, but having more users increases Mozilla's leverage over Google.
Anyway, your point isn't really relevant. Unless you believe Google is dictating nefarious things to Mozilla and has subverted Firefox (difficult since Firefox is open source, but not impossible), you should still be using Firefox. If you care about not continuing to give a giant, monopolistic advertising company control over the web, anyway.
The smaller Firefox's market share, the more Mitchell Baker gets paid.
EDIT: Except for MSFT... that would be interesting for sure.