I think it would have to be alternative search licenses, crowdsourced fundraising or some new product offering.
There's little precedent for crowdsourced fundraising on the scale of what Mozilla gets from its search deals. The examples I hear are Wikipedia and Tor. Wikipedia is the largest of its kind, I suspect the largest year-by-year internet crowdsourced project in existence and it gets half of what Mozilla's operations cost. Tor even less. So Mozilla would have to have more than double the largest crowdsourcing effort in history. So there goes crowdsourcing, at least as a primary option.
I don't think there's other licensing opportunities that pay out as much as Google, so there's strike two.
So then it's a matter of dabbling in side bets, which risk being inconsequential, compromising core mission values (e.g. adtech), or user backlash (VPN, Pocket) and stand accused of losing track of their mission. I personally wouldn't mind doing what Proton does and offering a drive/calendar/email suite but, again they would get accused of losing sight of their mission and I don't know how much they would stand to make from it. Nevertheless I do think continuing to experiment in side bets is worth trying.
Lastly, and perhaps most interestingly imo, they do some Ycombinator-style VC funding, which I believe are done out of the returns on their endowment. That might be one of the most intriguing directions over the long term, but again I have seen people on HN and point to that as yet another example of Mozilla supposedly failing to stay focused on their core mission.
Considering their existence depends on Google's money, it's to their best interest for the browser to lag behind Chrome.