In the worst scenario, if there is enough public interest to keep an industrial-grade kernel afloat, there is enough interest to keep an industrial-grade browser afloat. Firefox will survive Mozilla Corp like the original Mozilla suite survived Netscape.
TBH I don't think we'll ever get there. Mozilla just need to refocus a bit.
Possibly. The leading open source browser is currently Chromium though and its components are already extremely popular in the developer world. Node.js uses the V8 component. Electron, CEF, NWJS, and others use almost the whole thing for embeddable applications. Firefox even uses a lot of the open source components from Google like Skia. Many different companies and independent teams maintain customized versions of Chromium that are anything from settings customizations to large modifications.
So the question is not "is there going to be public interest to keep an industrial grade browser afloat" it's "is there enough developer interest to keep Firefox afloat when the thing that ate its lunch is open source".
And of course there are various levels of "afloat". Some forks of XUL Firefox are kept "afloat" as in they still compile but have no modern security model or major feature development.
The big thing though is open source developers never lost the opportunity to help FF achieve webrender or e10s or Quantum or add HDR support. Mozilla doesn't have to die before people can decide there is interest in keeping the browser afloat but it has slowly been dying for years. The only major feature I can think of that landed via community contribution was accelerated video decoding on Linux. Maybe that will change if Mozilla does go away but it's not a good sign it will lead to a truly "afloat" version.
TBH I don't think we'll ever get there. Mozilla just need to refocus a bit.