No, it doesn't. Firefox is a monorepo (of course, it's just one product). All their other products (bugzilla, rust, all the web properties) have different repos. Check out their other repos: https://github.com/mozilla
Many of those are synced back into mozilla-central (some bidirectionally, even).
> of course, it's just one product
In the sense that one package gets shipped to the end user, sure. But it's composed of many individual projects (SpiderMonkey, devtools, toolkit, layout...) which follow their own development paths and could each be broken out into their own repositories.