Firefox got a small update this week, pushing the version up to 1.5.0.3.  They have switched to automatic updating, so it is likely that you are already updated, but double check, as this is one of those nasty remote code execution issues.
“Malicious attackers could exploit the flaws to cause a denial-of-service attack, which in turn may allow them to take remote control of a user’s system, according to an alert from security research company Secunia, which rates the flaw as “highly critical.”

The flaws may be exploited when people attempt to engage a deleted component with designMode turned on. While this typically will crash the browser, it could also result in an attacker running malicious code, according to a Mozilla security advisory. Mozilla oversees the development of the Firefox browser.”

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