It seems unlikely that FF will ever be completely written in Rust. More likely that new bits that get added are Rust, and many of the old bits stay C++.
If FF gets rewritten in Rust, it will be a decades long effort.
I would just add that Mozilla also targets security critical portions with Rust rewrites. You don't necessarily need 100% Rust to be secure (if you have the right security model).
If FF gets rewritten in Rust, it will be a decades long effort.