This vulnerability is rated at low-risk and so far I'm not aware of any exploits of this in the wild.  A patch is most likely forthcoming and please be careful with all RSS feeds if you have the Sage extension implemented.

Firefox Sage Extension RSS Feed Script Insertion Vulnerability
http://secunia.com/advisories/21839/
http://www.frsirt.com/english/advisories/2006/3553

Advisory ID : FrSIRT/ADV-2006-3553
Rated as : Low Risk
Remotely Exploitable : Yes
Locally Exploitable : Yes
Release Date : 2006-09-11

A vulnerability has been identified in Sage (extension for Firefox), which could be exploited by attackers to execute arbitrary scripting code. This flaw is due to an input validation error when processing RSS feeds containing malformed data, which could be exploited by attackers to cause arbitrary scripting code to be executed by the user's browser in the security context of an affected Web site.

Solution:  Do not add RSS feeds from untrusted sources