"Unlike other browser flaws, this one is not subject to phishing or access to the system. But it can expose sensitive information from other Web sites you visited and the information you entered there," said Thomas Kristensen, Secunia chief technology officer.
While the flaw is only rated as "moderately critical" by Secunia, the rapid adoption of the open-source browser means that many users may be at risk. Prior to the release of version 1.0, downloads of earlier versions of the browser had reached 8 million within the first 18 months.
The Mozilla Foundation, which makes the Firefox browser, is working on a patch, and no cases have been reported, a representative for the group said.
Secunia has developed a test that allows people to see whether their system is affected by the vulnerability.