I remember reading something where Kaiser implied he's not particularly interested in learning Rust just because of FF development and therefore hasn't tried it, or something along those lines.Impossible as is right now. I actually did not know it was running on any PPC big endian platform. If Kaiser did not accomplish it, must have some real issues on PPC Mac OS X.
You'll probably need a GCC newer then 4.0 from 2005 to build a relatively new X11 also not from 2005, which is probably required if trying to build a newer FireFox using X11.
GCC 7 works, LLVM 7.1 works. No problems on compiler front either. Also, backporting once-existing support to a newer GCC is not an impossible task, especially as support has existed recently. IRIX hobbyists have 9.2.0 running and their support has been officially removed in 4.8.0.