Yeah, Apple seems to be able to fetch a little more than a billion per percent of the browser market (18% at 20B), but Mozilla is only able to score 0.5B for 2-3% of the market. Mozilla is getting a quarter of Apple’s rate.
That said, Apple has a lot more leverage than Google, and they can strong arm a better deal. I also wouldn’t be surprised if Safari users are just a more valuable marketing cohort. Firefox’s user base is going to have a lot more people who opt out of and or block targeted marketing.