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.
The use of a certain JavaScript instruction can cause Firefox to crash, allowing an attacker complete access to a system and the ability to run malware remotely.
Zalewski said that the attack could be carried out by convincing a user to access a specially-crafted HTML file that hosts JavaScript code targeting the vulnerability.
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.
The disclosure comes on the same day that Mozilla released an update for Firefox, which does not address the JavaScript flaw.
Mozilla and the US Computer Emergency Readiness Team urged users to mitigate the vulnerability by disabling JavaScript in Firefox.
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