It uses the Marian library via WASM (their wrapper for this is here) to do translations, which AFAICT is “AI” based (which I presume knocks the file size down quite a bit) - additionally, the language packs (I’m not sure what term to use here so I’ll just go with that) are not all bundled with Firefox, they’re downloaded when you first use them.

The previous incarnation of it, the Firefox extension’s repo was found over here - I assume the code is now within Firefox’s main repo since its built into Firefox now.