If you scale up IE9's share to allow for the proportion of Vista and Windows 7 usage, it's still only fractionally ahead of its open source competitor despite having been available for longer.
On the other hand, it is tempting to suggest that non-Microsoft browsers are more likely to be - though far from exclusively - used on computers that are not centrally managed. If that is the case, a slower buildup for IE9 would be expected while administrators carry out their own compatibility tests, and until the next versions of those organisations' standard operating environments are deployed.
Furthermore, IE9 has yet to be pushed out as an automatic update.
But none of this takes away from what must be seen as a very successful launch of Firefox 4.