From a purely business standpoint, the UI really is all that matters. Users don't choose firefox over Chrome just because FF has CSS cascade layers[0] (for example), they choose it because they think the sync/PW manager is better for them, or they might like the look of it more compared to alternatives.
That might well be why a user chooses it, even on iOS. But from my perspective as a business owner and developer, on iOS Chrome is Safari no matter what. When I test my site for use on iOS, I test in Safari. Which variant they use is irrelevant.
Firefox doesn't crash and pretty much works everywhere, and Safari doesn't crash (although it only works on Darwin); that doesn't sound like a differentiator.
0: https://caniuse.com/css-cascade-layers