I found something here:
http://www.arkom.co.uk/news-article.asp?NewsID=42
but it only pertains to sites run by arkom.
And like many other sites, they lump netscape and mozilla together as one, even though they are not one in the same. (Netscape 7 is built on mozilla 1.4).
Statmarket.com gets statistics from all of their clients, which is extensive, but you have to pay to see the stats.
there used to be a site called browserwatch, but its gone now, gobbled up by internet.com and scuttled.
google seems to find a lot of web site's stats pages that should not be spidered, but since they have no robots.txt file, google spiders them. It totally messes up the search on google for web browser usage or web browser statistics.