1. If the browser is an operating system (which it is) then the browser itself is naturally "ring 0". An example of where this matters is the fact that Hacker News can't access your bank account, while addons can. It is hard to imagine anything more security-sensitive than your bank account.

2. "An add-on is restricted to user privileges or less" doesn't mean much for the Firefox addons that are affected by this—they run with full user privileges, which for many attacks (malware, spying, TLS interception, keylogging, etc.) is close enough to ring 0 as to make no difference.

I'd argue the threat here is that malicious extensions at this level can be done by somebody with little to no experience, compared to kernel extensions which actually require some modicum of skill.

Requiring signing is a recognition of the fact we are increasingly moving our lives online, and hence into the browser.