As far as i know, the web rendering engine of Firefox in C++ were pretty old, coded in old c++ style. So in this particular case a rewrite would have some leverage.
The thing is, in the end, i dont know if this part C++, part Rust codebase wont start to create more problems than it solves.
Should the layout and rendering engine be rewritten in C++ or Rust? They decided to go with Rust, and now they will be forced to continue replacing C++ codebases with Rust, for consistency, workforce, etc..
In the end they will fight a lot to recode the browser, and meanwhile the competion can optimize, inovate and create more features.
Rust will gain a lot for sure, but not Firefox, not Mozilla. And with this Mozilla will be in a place where it will need to bet everything in Rust, as its only chance of survival.
End users dont care what technology go on their browsers, they care about perception, and its not clear that a browser will work better just because its in Rust as compared to C++ which is already a high performance language. (Maybe they will spend less resources in tests or in correcting bugs, but thats pretty much it)
Firefox is much better now than it was before? Yes, but my point is it could have been this better version of itself sooner, if they did not use Firefox to fight Rust´s cruzade to relevancy.