The vulnerability is currently unpatched, according to an advisory from Secunia. It is caused due to an error when processing JavaScript code handling e.g. font HTML tags and can be exploited to cause a memory corruption. Successful exploitation allows execution of arbitrary code. It has been confirmed in Firefox version 3.5 but other versions may also be affected, Secunia said. If you use Firefox, it s important to immediately disable JavaScript within the browser.
Source: https://threatpost.com/highly-critical-bug-bites-firefox-35-071409/72901/