Good to see an alternate Rust compiler being developed as more projects use it. See it isn't ready for 32bit x86 yet, hopefully will be some day especially if they follow through and have it outputting C code though I guess the assembler uses AS, same as GCC.
The problem with Firefox is lack of skilled developers. 52ESR will compile without Rust but I don't have the skills to build it. Things move so fast with Mozilla that every version is a big porting job. Be nice to have 52ESR working as there are enough forks that it could be usable for quite a while.