While it likely won't help with Firefox, need a lot more work then just Rust working, early Rust was very unstable with new incompatible regular releases.
What it will help is other programs that are compiled in Rust getting ported. Seems Rust is getting very popular and even small utilities are being ported to it.
Will also have to see how well it works on our 32 bit platform. Even current Mozilla doesn't compile if I have $TMPDIR pointed at an HPFS volume as at least one .S (assembly stage) temporary file is over 2GB's and likely it is the same file where GCC sometimes dies due to lack of memory. Sounds like Rust is more demanding then C++