"The net of all this is that any comparison of browser share adoption at this point is premature at best, and misleading at worst," Gavin said. "In a few months, we'll be better placed to look at the share of the latest browser versions and get a sense for relative progress and adoption."
According to Web metric company Net Applications, Firefox 4's average share for its first nine days was 3.2%, while IE9's in that same stretch was only 1.1%. On Wednesday, Firefox 4 passed the 4% share mark for the first time; the same day, IE9 stood at 1.4%.
It's not surprising that Firefox 4's share trumped IE9's. Unlike IE9, Mozilla's browser runs on Windows XP, Mac and Linux, operating systems that Microsoft's program doesn't support.
It was Microsoft's decision, however, to support IE9 on only Windows Vista and Windows 7, versions which collectively account for slightly more than a third of all copies of Windows now in use. The company has said it did not want to develop for what it called the "lowest common denominator," the 10-year-old Windows XP, because, said Gavin two weeks ago, "That's not what developers need to move the Web forward."
Gavin used that to make another point in his dispute with comparisons, arguing that since IE9 only runs on Vista and Windows 7 -- and is especially targeted at the latter -- its market share should be calculated solely on its use among Windows Vista and Windows 7 users.
"Adoption on Windows 7 is what we care about most," said Gavin. "Other browsers support other platforms, so if you want to draw comparisons you really need to take account of addressable base. With IE9, you essentially need to multiple by a factor of almost 3x to account for the difference in current addressable base."
Microsoft's made that case before, and in fact has gone so far as to claim that IE9 is the best browser for Windows because rivals "dilute" their energies on other operating systems.
Net Applications does not usually disclose granular data on its browser usage share numbers that would show how IE9 fares among Windows Vista and Windows 7 owners only, or how competitors like Firefox and Chrome match up on those operating systems.
The analytics company will release its March browser share data early Friday morning.