Firefox and Chrome already contain significant amounts of Rust code, and the proportion is increasing.

https://github.com/chromium/chromium : C++ 74.0%, Java 8.8%, Objective-C++ 4.8%, TypeScript 4.2%, HTML 2.5%, Python 2.4%, Other 3.3%

https://github.com/mozilla-firefox/firefox : JavaScript 28.9%, C++ 27.9%, HTML 21.8%, C 10.4%, Python 2.9%, Kotlin 2.7%, Other 5.4%

How significant?

According to https://4e6.github.io/firefox-lang-stats/, 12%.

I would love an updated version of https://docs.google.com/spreadsheets/d/1flUGg6Ut4bjtyWdyH_9e... (which stops in 2020).

For Chrome, I don't know if anyone has compiled the stats, but navigating from https://chromium.googlesource.com/chromium/src/+/refs/heads/... I see at least a bunch of vendored crates, so there's some use, which makes sense since in 2023 they announced that they would support it.