"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.
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...)