All the forks need to make a common engine independent of Mozilla. Pale Moon did it with Goanna and it is shared between Basilisk and K-Meleon as well. The big problem is that any new engine has to beat being filtered by Cloudflare or other WAFs that discriminate by user agent. A bold idea is for all the Firefox forks to rebase off of the new Ladybird engine and abandon the old Gecko codebase entirely.

There are a lot of decades old embedded systems out there. Every so often you hear about a big company still relying on floppy disks and other old tech, including major railways and airplane companies. Having the source code will help with debugging better than having to disassemble or other reverse engineering.