1. It’s not Mozilla’s job to guarantee that the 20 most popular add-on will work. It’s the job of the add-on developers.
2.
Case1: If add-on is known to be working on FF stable, it will not be blocked.
Case2: If an add-on is known to have problems in current FF, it will be blocked.
Case3: And here comes the change: if mozilla doesn’t know if the add-on works or not, it will be enabled and treated as a working one: until now, it was blocked.