Alexa ranks sites based primarily on tracking a sample set of Internet traffic - users of its toolbar for the Internet Explorer, Firefox and Google Chrome web browsers.The Alexa Toolbar includes a popup blocker, a search box, links to Amazon.com and the Alexa homepage, and the Alexa ranking of the site that the user is visiting. It also allows the user to rate the site and view links to external, relevant sites. In early 2005, Alexa stated that there had been 10 million downloads of the toolbar, though the company did not provide statistics about active usage. Originally, web pages were only ranked amongst users who had the Alexa Toolbar installed, and could be biased if a specific audience subgroup was reluctant to take part in the rankings. This caused some controversies over how representative Alexa's user base was of typical Internet behavior, especially for less-visited sites. In 2007, Michael Arrington provided examples of Alexa rankings known to contradict data from the comScore web analytics service, including ranking YouTube ahead of Google.
Until 2007, a third-party-supplied plugin for the Firefox browser served as the only option for Firefox users after Amazon abandoned its A9 toolbar. On July 16, 2007, Alexa released an official toolbar for Firefox called Sparky. On 16 April 2008, many users reported drastic shifts in their Alexa rankings. Alexa confirmed this later in the day with an announcement that they had released an updated ranking system, claiming that they would now take into account more sources of data "beyond Alexa Toolbar users".