Thats the stats for w3schools, no shit its going to be heading to FF. The people who are visiting the site are mostly technical. Why do people always site their stats to prove FF. FF does not have 40% of the market (although I wish they did).
I think the word you were looking for is 'cite', and perhaps they do that because it's the data that's available to them. If you want to get hyper-technical, it's childs play to spoof your user agent, so you can't really trust anybodies statistics.