Mozilla released a Firefox Heatmap which provides some insight into how they arrived at some of the Firefox 4.0 UI decisions (such as combining the stop and reload button or making the search button not button-y).
You can also glean some interesting conclusions by comparing users across Windows, OS X, and Linux. For example, the horizontal scrollbar is only used by 20.5% of OS X users vs 44.1% of Windows users. This could be attributed to either Macs coming with wider resolution screens or most Macs coming with mice/trackpads that allow built-in horizontal scrolling.
RSS Icon is clicked on by Linux users 13.8% vs 5.7% for Windows users. Linux users more tech savvy? Definitely.