Why do you think Firefox users would be most likely to make the jump? The main reason I see people give for supporting Ladybird is challenging the dominance of the incumbents. That's not really a great reason to switch from Firefox because, as you note, it doesn't have any dominance. And there's also an argument that splitting the non-Chrome market into two only increases Chrome's dominance.

From what I can tell from HN, Brave seems to be popular with those users who hate Google but for whatever reason hate Mozilla even more, and I suspect those will be the most likely users to switch.