Long time FF user, new Brave user. Turning off all the bloat in Brave makes it basically Chromium. Reason I'm switching is like FF, Brave has a self-hosted sync, however unlike Firefox, Brave has a tabbed user experience on Android tablets. If FF had that, I would not be switching.
No clue, but Brave was billed as a privacy focused browser. It regularly rates about the same as Firefox (by reviewers) for privacy unless you use custom FF configs which can go further. I've built and compiled both from scratch (contributing), and don't have concerns with either, though I prefer FF.