From the 'What Do You Click?' files:

Which button in the open source Firefox web browser is the one most clicked by users on Windows, Mac and Linux?

The answer to that question and more has been addressed in a new Mozilla Firefox Main Window Usage Study. The study data was collected on an opt-in basis from nearly 10,000 users of the Mozilla Test Pilot addon which surveys Firefox usage.

Mozilla developers aren't just looking to gather statistics for statistics sake either, the goal is to gain a better understanding of browser usage in an effort to provide a better experience in the upcoming Firefox 4 browser. In an effort to help visualize and display the data, Mozilla developers have also overlaid the study's data as a heatmap on top of a Firefox browser. A Heatmap graph visually indicates how active certain areas of a map - in this case a web browser - are in terms of usage.

So which button is the most clicked?

By a landslide the 'Back' button was the most clicked of all navigation buttons which include the Back, Forward, Reload, Stop, and Home buttons. Across Windows, Mac and Linux 93.1 percent of users clicked the button at least once over the course of a five-day period. In total the study reported that users clicked on the back button 66 times over the course of five days.