"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..."