Developer Michal Zalewski, who uncovered the flaw, described it as " seemingly pretty nasty, and apparently easily exploitable".
The vulnerability affects current versions of Firefox for all major PC platforms, according to Zalewski's report.
Bugzilla, the error-tracking system used by Mozilla, classifies the vulnerability as 'critical', the second-highest priority.
The vulnerability has only been demonstrated as a proof-of-concept code and there have been no reports of active exploits.
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