Why isn't it the same? The fact that it is possible to change that default means google simply pays less for it than they otherwise would if it wasn't changeable.

It's not a binary toggle - firefox is selling you as a source of revenue for themselves. They're just not making it as extreme as it is possible to be - in the hopes that you don't switch away.

You can compare same situation with safari in iOS. Except google pays a lot more, since you cannot switch away in iOS as easily, and culturally there's more reluctance compared to firefox users. This makes google pay more for iOS traffic, as those users are worth more.