Yeah, clang for all platforms for a couple years now

Some Linux distributions, like Fedora, build their Firefox binary with gcc.

Yeah but this will probably change too.

I hope not, we need diversity. (Yes, also more rust compilers)

We will still have jobs to build Firefox with gcc and gcc is not going anywhere in general: the % of package built with clang in Debian/Ubuntu is a fraction compared to gcc.

Nathan Froyd wrote this great blog post about compiler usage: https://blog.mozilla.org/nfroyd/2018/05/29/when-implementati...

Yes. There are essential Rust components in Firefox, and the only serious Rust compiler uses LLVM. (This has nothing to do with clang as someone else suggests)

"does firefox use llvm to compile" is a very weird question that can be interpreted in a variety of ways :D

Seems your interpretation is "does Firefox require any LLVM based compiler to compile?" and yeah. But "does Mozilla use clang for official builds?" is another valid way to parse the question.

(Mozilla does use clang, and they even do cross-language LTO thanks to that: https://blog.llvm.org/2019/09/closing-gap-cross-language-lto...)