It was on Mozilla.org before redirecting to getfirefox.com. Now, Mozilla has acquired Firefox.com and that’s the new canonical home.

See

https://news.ycombinator.com/item?id=37279788

When did Firefox Sync start using accounts.firefox.com for their login site? One assumes they’ve owned it since at least then, if not before. https://github.com/search?q=repo%3Amozilla-firefox%2Ffirefox... says 2014? So probably earlier than that, though I don't know for sure when.