Gecko-based browsers are, what, 1% of the browser market?
Last I checked, Firefox was close to 3%, but you right, still not a large number.
Guess which browser is the most targeted by malicious actors? Not the one having 1% of the market, that's for sure.
It doesn't have to attract the most attackers, especially if it is notoriously weak. You shouldn't choose software that is easier to exploit simply because it's more obscure. Do you also choose weaker encryption algorithms because no one uses them? I sure hope not.
And your comment would probably ring a little more true, except we just had the xz debacle, and guess how much Linux has of a market share? Probably close to Firefox's, both in the single digits.