They probably do. But Firefox addons is a long tail. There are many addons that would qualify as "popular" but Mozilla wouldn't be able to contact them all. So

They probably do. But Firefox addons is a long tail. There are many addons that would qualify as "popular" but Mozilla wouldn't be able to contact them all. So they have to measure not only popularity, but also if the community cares about that addon. I had many addons installed that I don't use, synchronized across devices: They may count as "popular" but it doesn't reflect the actual community and usage.