Become a non-profit. If GNOME can alienate all of its users yet still be the de facto default desktop Mozilla can figure out how to do whatever everyone else is doing abd just do it.
Honestly, I'm not sure how GNOME manages to be, and remain, the de-facto default desktop for SO many distros. It's alienated so many users with its, well, opinionated approach to things, causing not one but multiple forks and competing projects (Cinnamon, MATE, etc.), in addition to other competitors (KDE, xfce, lxde, lxQt, etc.). Why SO many distros still feature GNOME as the default, instead of trying to differentiate themselves and push a different default like KDE, I really don't know, but I suspect it involves some kind of politics within the Linux distro realm. Perhaps there's still some crazy idea about it being more "newbie-friendly", but it's been decades now and Linux on the desktop has barely made any inroads, so I think it's really time to give up on that and focus on users who actually like Linux and are already experienced.
Anyway, as I wrote before, I don't think this is something you can just replicate with Mozilla/Firefox; it's a completely different situation, and I don't think GNOME's apparent success holds any lessons elsewhere.