There is a critical JavaScript vulnerability in the Firefox 3.5 Web browser, Mozilla has warned.

The zero-day flaw lies in Firefox 3.5's Just-in-time (JIT) JavaScript compiler. Proof-of-concept code to exploit the vulnerability has been posted online by a security research group, Mozilla said in a post on its security blog on Wednesday. Security company Secunia rated the vulnerability as "highly critical" on Wednesday.

The hole could allow a hacker to launch a "drive-by" attack, according to...