At least for my site, we consider multiple channels of engagement when calculating whether a user who is subscribed to something is active enough to bother mess

At least for my site, we consider multiple channels of engagement when calculating whether a user who is subscribed to something is active enough to bother messaging. If you don't click/open the emails (as far as our tracking can tell), but your account still opens our app, you'll keep on getting email updates until you unsubscribe via the unsub link, the profile preferences, or you close your account. Mozilla is probably in a bit weirder position because their value-prop isn't in curating articles and videos the user is interested in. They don't require the users to login to use Firefox, and thus can't know if text-only users are using their emails unless they process unique ids on links inside the email (and that's the last step of the usual funnel, which means the segment definition is going to have to be really long (probably a year)).