I'd much rather see Firefox use miTLS[0]. They already use HACL[1] (miTLS and HACL are part of Project Everest[2]) in NSS[3], so I don't think it would be too big of a leap. Rust is really cool, but if you want to ensure that the implementation adheres to the spec, a language with formal verification is much better. That said, miTLS isn't finished yet, so of course it's going to take a while until anyone can actually use it.
[1] https://github.com/project-everest/hacl-star
[2] https://project-everest.github.io/
[3] https://blog.mozilla.org/security/2017/09/13/verified-crypto...