A possible alternative source of income for Mozilla I think would be, and one I haven’t seen mentioned before, providing consulting services in their areas of expertise.

For example, demand for Rust engineers seems to be steadily increasing[1] and I can’t think of a better company with a larger concentration of Rust expertise than Mozilla. Perhaps the same could work for other areas (ex frontend), but I’m biased toward Rust. Due to lack of experience in this regard I’m not sure if this could actually work, but I could imagine allocating part of their engineering time budget to consultation, whose income would fund their core products.

[1] Besides all the articles popping up about starting to employ Rust by well known companies, my anecdote as a full-time Rust SDE is that I’m getting more and more requests on LinkedIn, and not just for the usual crypto roles.