None of them are a guarantee for it to survive but diversity is important and hard to see a second option disappearing just like how BSD are surviving.
Imagine what kind of HN thread pops up if Firefox is abandoned by Mozilla.
Also Firefox is a single licensed product that is MPL, and for anyone who wants it that way commercially may support it.
I see the main asset of firefox as being
a) not google tech
b) mobile browser with ublock
c) tech nerds with tons of custom FF addons, never wanting to switch
So I don't see them go away anytime soon, but I also see nothing to stop the decrease of their marketshare. That means, unless mozilla would focus on their core values again. They lost much trust with quite some shady actions now, over the years.