Microsoft's Internet Explorer's market share is absolutely falling. The question is, by how much?

I've reported before that Internet Explorer (IE) drops 5 percent market share points each year, while Mozilla Firefox gains 5 percentage points per year. But what is becoming increasingly clear is that IE's market share may be dropping more precipitously than previously reported, falling to 60 percent share in June 2009 instead of the 68 percent share expected.

Or is it?

The answer may depend on the source of the information, and the reliability of its data. Mozilla's Asa Dotzler uses StatCounter data to discern a 60 percent share for IE but, as ZDNet's Larry Dignan points out, this data may not hold up.

For Microsoft's sake, it had better hope not, as this chart compiled by Dotzler shows:

Internet Explorer market share falling faster than reported?

(Credit: Asa Dotzler (Data from StatCounter))

That's not the sort of chart with which Microsoft CEO Steve Ballmer likes to sweeten his coffee in the morning.

Net Applications, the other big source of browser market share data, still hasn't posted its results for June 2009, noting that it is trying to make sense of "some significant variations in browser and operating system statistics."

Given that market share data isn't a one-month phenomenon, it's not necessarily helpful to celebrate or fret over the June data, especially since much of the market share share data is going to get skewed in the summer months, anyway. For example, given Firefox's disproportionately large following in Europe, coupled with Europe's disproportionately long holiday season in the summer, I'd expect to see Firefox drop some percentage points against IE through August, only to rebound strongly in September.

Regardless of short-term variations, one thing seems clear: Firefox is gaining on IE. Microsoft spent too long enjoying its browser dominance, and not enough time innovating. It's starting to pump R&D dollars into IE again, but it's not yet clear whether its monolithic approach to browser development can compete in the long term with Mozilla's community-developed Firefox.

Microsoft needs to compete again, or risks seeing even StatCounter's data understate just how quickly it's falling.

Mozilla, for its part, faces a host of new challenges. It can't afford to waste much time with back slaps and high-fives. The browser has become the center of computing. Microsoft isn't going to give up easily, nor will Google or Apple.