Hence the crazy idea of now using WebAssembly in Firefox modules instead.
Not to mention how much less experience you need in Rust to not blow everybody's feet off by mistake. I reckon if you have 10 years C++ and six months Rust, any Rust you write is already more likely to deliver reasonable performance without setting everything on fire than your C++. Because of the constant exposure to outright malevolent stinking garbage (in the form of other people's HTML, CSS and Javascript) the browser needs to be exceptionally robust, and C++ just isn't very good for that. So Rust is often a better fit for what Mozilla do.
Chrome folks have been playing with Rust, but seem more keen in improving their C++ static analysis tooling instead.
As for Mozilla, it is 10% of Rust code and lets see for how long Firefox still matters, given the existing 3% market, even EdgeChrome has surpassed it.
I would say that at this point that's good money after bad. Linus of course also put a bunch of effort into static analysis, that's what "sparse" is.
The thing you run into immediately is that your programming language doesn't express the thing you wanted to analyse very well. So you have to annotate your software (Linux is sprinkled with sparse annotations), and now you've added an extra opportunity for mistakes, because the annotations are transparent to the compiler, so you can write code which analyses as correct but compiles to something incorrect. "Hooray".
Most of that code is surely related to what was replaced initially instead of new subsystems being ported.
Irrelevant to the question of how much Rust is in Firefox.
> is surely
Okay, it's clear you're not going to change your mind, no matter what.
What Mozilla did do was lay off many of the people working on Rust itself as their full-time job, as opposed to people who use Rust to do their work at Mozilla. And the Servo team, unfortunately.
The 10% as pointed out in a sibling comment, while admirable isn't large parts.
As you can see from the breakdown someone posted [0], almost two-thirds of the Firefox codebase is HTML / JavaScript / Python (for tests) / assembly / Java (for Android), none of which is a candidate for being rewritten in Rust to begin with.
If you just look at the portion written in native system languages, Rust is slightly more than 1/3 of that code already and still climbing.