Great news. With the current system, firefox's stability, upgrade compatibility and performance were entirely dependent on every author of every extension I use being sufficiently careful. It meant a lot of periodic purges, housekeeping, and reduced trust. I'm sure the overhead helped users move to chrome, despite Firefox's vanilla install being lighter. Reducing the API surface was a necessity.

Also, it takes a lot of effort to cope with the churn in internal APIs, and I had to get rid of promising extensions like Pentadactyl because they broke too often. With a smaller, stable API, that problem doesn't exist. I don't believe that the current tradeoff of power vs responsibility was working out in the majority of cases, and I've seen enough evidence in the form of lagging addons and neglected ports.