The Mozilla people seem to be quite good with the stats. I think they've looked at the whole user "funnel" (i.e. from website to download to install to continued use) and focused on the parts they can get the biggest wins. Note the blog title: Blog of Metrics
Though perhaps they've got a blindspot in that they're more comfortable making changes in desktop code and so started there.
Mozilla had a lot of people focused on other areas who were trying to do ad-hoc analysis on the side. That is why our team was formed. The metrics team set about to measure and improve user interaction with certain critical pages like this IE landing page, and we discovered that it hadn't received the same level of attention that certain other pages had in regards to optimization.
Though perhaps they've got a blindspot in that they're more comfortable making changes in desktop code and so started there.