Because I’m sure there are a lot of Firefox fans all over the Internet, I must disappoint you and report another vulnerability discovered in the Mozilla browser that can allow an attacker to
compromise an affected computer. No matter the version of the application, Firefox is affected by a highly critical security flaw due to a vulnerability discovered in Firebug, a Mozilla browser extension. If you’re already using the add-on, then you should know that Firebug is a JavaScript debugger with useful features like script explorer, dynamic console and CSS viewer and editor.

Security company Secunia discovered the vulnerability in all the versions released before the current 1.02, adding that the flaw is highly critical and all the users must update to the latest version of the extension. “Firebug does not properly sanitize input passed to the "console.log()" function. This can be exploited to e.g. execute arbitrary script code within the "chrome:" context by tricking a user into visiting a malicious website,” Secunia sustained in the security advisory.
http://news.softpedia.com/news/Secur...er-51374.shtml
Security Flaw + Firefox = Evil Browser - Mozilla’s browser is affected by a critical vulnerability - Softpedia