Firefox developers have released a security update that they hope will fix a lingering security threat in the popular open source browser.

The Firefox 2.0.0.6 update fixes a problem in the way Firefox handles the uniform resource indicators (URI) that are used to launch other applications when an unsupported file type is loaded.

The URI vulnerability was originally discovered as a "cross-browser" flaw in which an attacker could use a specially crafted internet address in Internet Explorer to launch Firefox without security protections and run malicious code.

Mozilla had attempted to fix the flaw in the 2.0.0.5 update, claiming that any further fixes would be the responsibility of Microsoft.

Following the 2.0.0.5 release, however, security researcher Jesper Johansson pointed out that the URI handler in Firefox remained just as vulnerable as Internet Explorer. The problem, noted Johannson, was that Firefox did not properly format the URI address, allowing an attacker to potentially insert multiple malicious instructions.

In addition to fixing the way URI addresses are displayed, the 2.0.0.6 update also repairs a vulnerability that would allow an attacker escalate privileges through a specially crafted about:blank window. That vulnerability is listed as "moderate," the second of Mozilla's four alert levels.

No other security or performance fixes were included in the update.