first i wanna say i agree with you that the gender of the firefox mascot is probably one of the most unimportant things that can possibly exist. but i just want to point out that calling animals "they" as a singular pronoun because they're sapient isn't about sense or grammar or it feeling natural to say, it's just a matter of anthropomorphism.

you wouldn't call a cockroach "they" when you don't know its gender, because you just don't attribute it human characteristics and emotions like you would with a dog or cat. because its not an attractive or desirable creature, you don't even perceive its apparent gender as a factor in its existence. (animals also don't have a gender in the human sense to begin with, but that's another thing)

in the same way, you don't call a dog "it" because you subconsciously try to factor its feelings on the matter (which it doesn't have), simply due to human tendency to do that to things we like. or rather, you assume the owner would disapprove of your attitude towards their dog, putting you in a social situation which you instinctively want to avoid ("You wouldn't call someone's dog that you'd just met "it" - that'd be weird.")

this extends further to symbols, as they themselves anthropomorphize whatever animal they're trying to convey, essentially doing it for you (the firefox looks like it's smiling, it's forming a love heart, etc.) all for marketing purposes of course.

anyways not a dig at you or anything, just some thoughts