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.
