And with great power comes great responsibility.
Security between addons isn't an issue, since they're intentionally not sandboxed from each other (and that wouldn't even make sense). Keeping them isolated from web pages is simpler, since that's already required for core browser functionality. The biggest issue is making sure addons themselves don't expose the user (such as Greasemonkey's unsafeWindow), but again, that comes at the expense of the power that addons can wield.
at the expense of requiring add-ons to be rewritten.
This would kill Firefox, so they will never do it, and I'm fine with that. We would undoubtedly get something worse than we have now (e.g., Chrome's limitations).