who use *NIX properly with a (non-priv'd) user.

Worst thing that can happen is a problem with the users directory.

OH and BTW

Successful exploitation requires that the site is allowed to install software (default sites are "update.mozilla.org" and "addons.mozilla.org").

A combination of vulnerability 1 and 2 can be exploited to execute arbitrary code.

There are other exploits/vulnerabilities that are this minimal as well on *NIX.

Windows this is extremely bad, the one size fits all and "open-m^Hness" that Windows is... well we know why it is a problem.