Google being Apple's customer doesn't mean Google isn't sponsoring Mozilla.
These are two very different companies with a very different relationship with Google. And very different influences on Google.
Google wants to be on iOS. It brings customers to Google. A lot of them. iOS is possibly more profitable to Google than Android even with all the payments Apple extracts from them.
Google needs Mozilla so that Google may pretend that there's competition in browser space and that they don't own standards committees. The latter already isn't really true, and Google increasingly doesn't care about the former.
Google isn't a sponsor of Mozilla, they're a customer. Do people think Google is "sponsoring" Apple with $1.5 billion a year too?