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Mozilla is aware of and has publicly acknowledged the issue on their blog. They say that the bug can be exploited by an attacker who tricks a victim into viewing a malicious Web page containing the exploit code. Mozilla is working to fix the issue and a security update will be sent out when it's ready.
If that sounds a bit too troublesome, you can simply run Firefox in Safe Mode or even install an add-on like NoScript. Naturally, as soon as the fix is released, you can reverse any remedy.