Sustainability wise, Firefox is not built for hackers. One example is the Haiku community has been working on porting Firefox for years, to varying degrees of success. The problem is that it's just so huge; there are so many things going on, so many libraries that each require porting efforts on their own, DRM standards built into the core platform that will likely never see support, so real "Firefox" is likely to never happen regardless of community effort.
Meanwhile, though it is in a quite early state, multiple people are already experimenting with their own working builds of Ladybird on Haiku.
I'm not trying to say I can guarantee this will continue being the case as the ladybird project reaches maturity, but I think they deserve an honest shot. Wouldn't it be nice to have a truly portable browser engine?