Servo components are being uplifted into Gecko gradually. There's less benefit to rewriting the JIT in Rust because static type systems can only do so much when the whole goal of a program is to generate code dynamically.
My suggestion was to have Firefox support both Gecko (with uplifted component) and a pure Rust renderer, with the latter to be used in the Tor mode where security (to preserve anonymity against resourceful adversaries) matters over compatibility.
For security, instead of a JIT, a simple JavaScript bytecode interpreter written in Rust to be used exclusively in the Tor mode would be ideal, for maximum security at the cost of worse performance.
Another option is a JIT that generates code that is easily proven to be safe (e.g. because it does a bounds check on all memory accesses and only does indirect jumps using a jump table, or because it's the only thing running in a process and jumps are still constrained with a read only jump table and read only code).
