[Yaina]

Firefox is still largely C++ and Javascript and that's not going to change anytime soon. But it makes sense, because mostly you're adding or changing stuff that's already deeply connected with all the existing code.

I think you see Rust when there are really big projects that can be treated as a library. For instance the network team is writing their QUIC implementation in Rust – called neqo – and I can imagine that this was possible because it's low level enough and separate enough from the rest of Firefox.

It's of course also a resourcing question. For instance the download-manager code in Firefox is a big, big mess and it's not really owned by anyone. But it works, is safe, and handles a lot edge cases. Rewriting this thing (even again in Javascript) would be an enormous effort, and that's true for so many parts of the code base that might benefit from rewrites. A rewrite would be nice, but there probably will always be more important things to work on.