What would be an acceptable revenue model? Google Chrome has the same privacy profile with the exception that Google retains the data for their own ad platforms.
Selling preferential search access is legally precarious due to FTC's lawsuit against Mozilla.
Wait the FTC is suing Mozilla?
> What would be an acceptable revenue model?
They could start with the one they've refused for ages even though many have asked for it. Let people directly donate to fund the development of firefox (as opposed to just giving mozilla money to funnel into any number of their other projects). They could even make money selling merch if they didn't tank the brand. Firefox could have a very nice niche to fill as a privacy focused browser for power users who desire customization and security, but sadly they don't seem interested in being that. For whatever reason they'd rather spend a fortune buying adtech from facebook employees and be a chrome clone that pushes ads and sells user data, and that isn't going to inspire support from users.
That said, I'm not convinced that every open source project needs to be profit generating. Many projects are hugely successful without resorting to ads. What makes it possible for VLC or even Arch Linux to thrive without advertising that couldn't work just as well for firefox? The solution is certainly not to turn Firefox into a project that their users no longer want to support or use at all, but that seems to be where they are headed by selling out their userbase.