Let me explain the angle that I am coming at this from:

I am trans. I am not specifically non-binary, but I appreciate all types of trans and non-binary representation in media, and want to support it. I appreciate it because it shows that the creator is willing to stand up for my people, and because it helps to normalize our existence to the audience of the media. Both of those reasons depend on how explicit the representation is, because both depend on the average viewer of the media being aware of the character being trans and/or non-binary.

To me, this does not seem like explicit non-binary representation at all. Kit could be considered to be whatever gender the user wants, maybe even subconsciously. I don't think Mozilla's intention is for it to be subtle non-binary representation, either, I think that their intention is for users to be able to view Kit however they want - which would be a binary gender in most cases. They leave the door open for non-binary users to see Kit as non-binary too, which is better than being openly hostile, but in the current environment trans and non-binary people need actual allies to push back against the rampant hostility they face. So I don't see this as a reason to specifically support Firefox. They're just doing the bare minimum by not being openly hostile.

To contrast with that, take the mascot of Honkai: Star Rail, Pom-Pom. The developer, MiHoYo, is located in China, so they are legally barred from showing explicit LGBTQ+ representation. However, they go right up to that line and even arguably over it on many occasions, going basically as far as they can without saying it explicitly. Pom-Pom is no exception. All of the characters in the game refer to Pom-Pom with they/them pronouns specifically, including characters who know Pom-Pom very well. That goes far beyond what Mozilla did with Kit, because the implication that Pom-Pom is enby is quite clear and consistent. Mozilla never once used they/them pronouns to refer to Kit from what I have seen, and Mozilla could say outright that Kit is non-binary if they wanted to, so them not doing so is a choice.