You can also use the Nightly app and load addon collections. It's an absolute pain in the arse, and a lot of extensions don't work, but it's one way to get them.
Honestly, it's as if they don't really want you using it. It's a legitimate feature but hidden away and quite difficult to use. Here's a guide on getting it set up. I've been using it since it came out and haven't had too many issues, but a lot of addons just don't work as intended.