If users can inject <script> tags, I don't see any reason why they shouldn't be able to inject </csp> tags.
What Mozilla proposed is not a solution either, though: there is no reason users cannot upload scripts to a host. Heck, websites on most free webhosts would still be vulnerable. I think a better solution would be to only allow javascript to exist in the <head> tag, although this wouldn't be a solution for websites which, for whatever weird reason, would allow users to put stuff in the <head> of a webpage. XSS is an inherent problem with launching scripts from editable content. It would be better to disallow <script> tags completely and use a different mechanism to use javascript on a webpage alongside the HTML delivery, but that would mean properties like onclick and onmouseover would also have to be banned, which induces some rather serious limitations (read: webmasters won't like this).
For now, I'll stick with bbcode and some url encoding.
ps: wow, "webmasters". when was the last time I used that phrase?