I’m AFK but, almost certainly.

Most is in media codecs (like libvpx, dav1d and jpeglib-turbo), some is in nss (encryption), and some is in the JS engine (part of libffi, enabling privileged javascript code to load and call into non-js library code) and a tiny bit in nspr ("portable runtime", to implement things like platform-specific AtomicIncrement).

The actual JS JIT will produce platform specific low level code, but it's not really written in asm (other than some minor snippets maybe), so it doesn't really show up in such LoC stats. The engine itself is mostly C++, but if I remember correctly some Rust too in specific places (like a wasm compiler).