It's just a vectorised image of Yukimura from Samurai Warriors[1]. This image to be specific[2]. Don't think there was a designer, just a developer making a logo using a character they like (or maybe it was random image search). Pretty common in open source.
Isn't that just straight up copyright infringement? I understand that it's common in developer tooling to pick a character and use them as a logo, but it seems like a bold move to do that with some sort of external tooling at Mozilla's scale.
