It's not a sponsorship, it's a sale.It's also made such deals with Yandex, Baidu, and Yahoo before. The sale is for eyeballs on your search (ad) service. Firefox is getting in a precarious situation with it though as the number of users on Firefox has been decreasing (in absolute terms) despite the number of web users increasing.
They can do that for as long as Google is willing to pay. Without additional revenue stream, the day Google decides to cut cost and stop sponsoring Mozilla, that's the day Firefox will run into big trouble. Any additional revenue stream is going to help.
I am no CEO but that seems very clear to me.