Probably not much: the requirement is exact equivalence of program inputs to outputs, and as such the agents are performing very mechanical translation from the existing C++ code to Rust. Their prompts aren't "implement X browser component in rust", they're "translate this C++ code to Rust, with these extra details that you can't glean from the code itself."

It's like only 10% of Firefox is rust.

I wonder where did you get the idea that Firefox was all Rust. Made me curious.

Only a small portion of Firefox is written in Rust. Apparently some of the most performant and least buggy parts are those in Rust, but again, only parts like the CSS engine.

https://github.com/mozilla-firefox/firefox Rust isn't even mentioned in languages used.