This week, a set of vulnerabilities that could potentially affect both Internet Explorer and Firefox have been discovered. While not listed as serious threats, with a relatively small chance of data compromise, the vulnerabilities are unique in that there is potential for them to affect multiple browsers. In particular, the vulnerabilities would let someone read information from one browser window within another or install rogue software. The threats are not considered serious, with both being classified as "Less Critical":

"We consider this flaw less serious than the other IE flaw," Ijzerman said. "A user would have to have multiple browsers open, and the information on the site would have to be relevant to what the attacker wanted."
Microsoft says they have no reports of actual compromise via these flaws. If you are interested, there are places you can use to probe your own browser for these vulnerabilities.