1. Representing the public interest wrt Web -- especially fervent respect for privacy in the browser, almost as much emphasis on security, fighting commercial excesses around de jure and defacto standards, and social equity within the scope of what Mozilla does (e.g., software accessibility, encouraging all volunteers).
2. Competitive browser compliance and performance. To be viable as a daily driver, to have impact.
3. Forward-looking R&D on Internet public interest things not currently considered "browser", but potentially big in near term.
4. Fancy browser features that actually increase uptake and retention. (Much lower than the first 3.)
5. Designer's opinion/portfolio UX changes. (Could be improvements, and there's value in an appealing and moving-target modern appearance, but misalignment is a risk to be careful of.)
6. Necessary-evil money-making compromises. (Very, very carefully. Many past ones were negatives. And we still see arguable dark patterns around some of those, such as as Firefox updates sometimes reverting users' express Search-related preferences.)
A marketing challenge of these priorities would be that even most techies can't assess how well you're doing #1.
But that mix of things you do can include things understandable to your typical user, such as superior blocking of abusive ads. (But don't drop the ball on things that your typical user can't understand -- they're entrusting you as an expert to act in their interests.)
Educational outreaches can also help. And if you get to the point that you're generating an order of magnitude fewer security vulnerabilities than your competitor, then marketing people can run with that. But be careful with positioning, so that communications about smart, hard activities don't get conflated in people's minds with confusing mission drift and fluffy PR.