From what I read Mozilla fired one or more Rust developers related to the JIT or the interpreter and IMO the JIT should have been the first thing rewritten in Rust since that is one of the components that run arbitrary code. I really hope they can finish the port and I can't wait to see how many zero days will it have.

most of the JIT bugs were probably due to generating unsafe code instead of generating from unsafe code. Rewriting a new JIT will probably increase the number of zero days.