I think that's a bit unfair to Mozilla. They previously had prioritized extensibility over security, that is, extensions and plugins had basically direct access

I think that's a bit unfair to Mozilla. They previously had prioritized extensibility over security, that is, extensions and plugins had basically direct access to the browser's C code itself. They've now prioritized security over extensibility. That's not to say that extensibility isn't a priority (or that security was previously not a priority).