> Because safety. Mozilla and others are just making the Web browsers safer.
And yet they broke things like NoScript, which provided security/privacy features missing from plain Firefox and unavailable on any other browser no matter what extensions you installed. If safety was really such a high priority, they would have waited until after a full-featured NoScript WebExtension was possible to implement before deliberately breaking XUL extensions.
It was necessary the Internet and explosion of security exploits to realize that although plugins in-process are easier to implement, the older out-of-process architecture was safer and harder to exploit.