McAfee has issued a warning about a Trojan virus that disguises itself as the popular 'Numberedlinks' Firefox extension. Once installed, it logs all the credit card information that you enter in your browser as well as all of your passwords. It then sends this information to a server.
The scary part is that this extension does not install itself through the Extension Manager (which requires an explicit confirmation by the user). Instead, this plugin directly copies itself to the Firefox configuration data.
More info here.