The focus shouldn't be Firefox per se, it should be Gecko.
In this market, we've got an army of secondary secondary browser players-- the Vivaldis, the Braves, Yandex's browser team, the smouldering wreckage of Opera, and yes even Microsoft's Edge and (to a lesser extent) Apple's Safari teams.
I'm sure every single one of those developers knows they're living with a belligerent landlord: they're going to spend too much of their careers having to unwind anti-competitive or just "we wouldn't want to include this in OUR browser" features in Blink.
Make sure you're a serious option for them. Make Gecko as embeddable as Webkit.
In this plan, the endgame actually has a fairly low Firefox market share, because it gradually evolves to be the "packed-in reference implementation" of the Gecko engine. Instead, we focused on building a healthier web ecosystem. More people end up using Gecko!Edge or Gecko!Vivaldi or whatever, and this breaks up market share enough that Google can't barge their way through the standards process by unilaterally cramming features into Blink. The individual vendors can more completely focus on their custom improvements while expecting that the core browser engine isn't going to be polluted to serve a specific (and not their own) commercial interest, and have a vested commercial interest in supporting Mozilla to do the heavy lifting for them.
In the short term, I think maybe they could start appealing to this market as a "too important to fail" concept. Think of AMD 10 years ago: the products are mediocre right now, but everyone knew if they went bankrupt, the industry would stagnate majorly for a long time. That might be a justification for soliciting foundation memberships or hackathons to encourage development work on their engine.