"Plug-ins allow you to heighten your Web browsing experience by letting you easily install programs to use as part of your Web browser. However, what makes a plug-in safe to use? There is, indeed, an issue with establishing the security, veracity and stability of Firefox and other Mozilla product extensions, now known as add-ons. For example, in July of 2006 a fake Firefox add-on was discovered to have attempted to steal passwords and perform key logging after installation. This add-on was particularly worrying because it was capable of being installed without prompting the user.
"Unfortunately, there isn't a ready way to determine whether an extension is malicious..."