There's a big difference between "We don't employ the language's architects" (so far as I'm aware Mozilla also doesn't employ many WG21 members) and "We don't have any engineers who know this language". In 2022 you'd probably have to go out of your way to hire that many engineers and not get some people who know Rust.
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.
Yet Chrome and Safari, the browsers that really matter in 2022, won't be moving away from C++.
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.
> Chrome folks have been playing with Rust, but seem more keen in improving their C++ static analysis tooling instead.
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".
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.