Market Browser Share by Month (by w3schools) - however, Net Applications is the standard measurement for market share. Since it's a fee-based service, check out this recent article in PC World. IE users make up 59.95% of the share, so, my advice would be tweak it until it works. ;)
Though, I guess another point to consider is that Firefox does seem to be more popular with roleplayers, for all the add-ons!