There are a few things this heat-map is not showing that are pretty important and may negatively affect their redesign decisions.<p>1) Add-Ons.
If we have add-ons that make other areas of the browser clickable, and those areas are highly used by a high number of add-on users, then that too may need to be taken into consideration on the redesign.
Example: a downloads bar at the bottom of the page, or maybe firebug.<p>2) Visuals that may or may not be turned on. For example, the X on each of the tabs. I close out tabs a lot, and that little X I have next to each tab is probably used more than any other button.<p>3) Custom settings. I guess this doesn't matter as much, but I, for example, have my settings in such a way that more tabs display across the top than the default install, so I never use that annoying down arrow on the right.