Mozilla further warned that a working exploit has been publically released, increasing the risk of attacks occurring in the wild.
Users can also reduce the risk of attack by running the browser in Windows Safe Mode.
The flaw is the latest in a string of high-profile browser exploits in recent days. Last week Microsoft warned of a flaw in a video ActiveX plug-in that was actively being targeted in Internet Explorer, and yesterday the company reported a second vulnerable IE component, this time an Office plug-in, that was being targeted by attackers.
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