Firefox extensions --- caution

... only Adblock Plus was found to contain no flaws that could be exploited by a malicious add-on that relied on reuse vulnerabilities. Besides NoScript, Video DownloadHelper, Firebug, Greasemonkey, and FlashGot Mass Down all contained bugs that ... made it possible to steal browser cookies, control or access a computer's file system, or to open webpages to sites of an attacker's choosing.

The researchers noted that attackers must clear several hurdles for their malicious add-on to succeed. First, someone must go through the trouble of installing the trojanized extension. Second, the computer that downloads it must have enough vulnerable third-party add-ons installed to achieve the attackers' objective. Still, the abundance of vulnerable add-ons makes the odds favor attackers, at least in many scenarios.