FireFox users beware, a rogue extension dubbed FormSpy is out to steal private information. Disguising itself as the very legitimate NumberLinks extension, the malware will actively look for passwords and private information like credit card numbers, as well as monitoring things like FTP and email traffic for private data. Primarily, a user gets infected by this extension through a piece of 3rd party software called Downloader-AXM. While that particular piece of software only affects Windows users, the fact that malicious extensions exist goes to show that ultimately, no matter how much security a system has, the user still has to make up their mind as to what they allow on their machines. You can read McAfee's security response here.