It's entirely possible that Google won't renew the deal, which would put about $100 million of Firefox's revenue at stake.
It would make no sense for Google to renew this agreement... if Mozilla suddenly changed Firefox's default SE that would not go over well with the users. It seems much more likely that they would form a new deal with less revenue going to Mozilla.
Of course that doesn't mean Firefox is done. That is highly dependent on Mozilla's innovation in the next few years.