The aesthetics of the FF addon marketplace are the way they are because Mozilla neglected that property for many years. There's currently a new initiative to redesign and build out an updated version of the marketplace, so expect the design to look like the chrome one soon.
I was purposefully judging the book by it's cover, because I was talking strictly about the cover. I know FireFox is a brilliant piece of engineering and I never mentioned Mozilla. Red Hat spends a lot on Linux development too, doesn't mean anything about the aesthetics of the platform.