Despite IE9's head start, Firefox 4 usage is already outstripping that of Microsoft's latest browser according to one web analytics firm.
Figures published by StatCounter show that Firefox 4 achieved a higher global web share on its launch day than IE9 managed after a week of general availability.
The actual numbers were 1.95% for Firefox 4 and 0.87% for IE9. These figures include any beta, preview and similar versions still in use.
"Firefox 4 has really hit the ground running and has eclipsed the launch of IE 9," said StatCounter CEO Aodhan Cullen. "Although IE 9 has undoubtedly been hampered by its lack of compatibility with Windows XP."
Is XP compatibility really that big a deal? Please read on.
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.