Alexa ranks sites based on visits from users of its Alexa Toolbar for Internet Explorer and from integrated sidebars in Mozilla and Netscape. In addition to their own statusbar extension, Sparky, there are several third-party extensions for Mozilla Firefox:

There is some controversy over how representative Alexa's user base is of typical Internet behavior. If Alexa's user base is a fair statistical sample of the Internet user population, Alexa's ranking should be quite accurate. In reality, not much is known about the sample and possible sampling biases.