Mozilla: "Firefox's current overall market share is 20%"

Every time I post NetApplications data on this blog several of you go to great lengths to point out to me that it's a flawed metric, especially when it comes to the market share for the browser Firefox. For one reason or another, NetApplications is wrong/bias/too US-centric/controlled by Microsoft* (delete as appropriate).

With this in mind I approached Mozilla PR and asked then a straight question:

Me: Quick question - what does Mozilla consider the overall Firefox market share to be?

Within a few minutes I got back a straight answer:

Mozilla PR: Firefox's current overall market share is 20%.

Hmmm, I thought, that's damn close to the market share that NetApplications posted for Firefox. In fact, just the other day NetApplications published a featured report that said that Firefox had hit 20% for two months in October.

So I asked another question:

Me: What do you folks base that on?

To which I got the prompt reply:

Mozilla PR: We use Net Applications to come help come up with these numbers.

So, the takeaway there is that if the folks at Mozilla are happy to use (and quote) NetApplications data for Firefox, then I'm betting that the overall market share isn't going to be much above that, because if Mozilla had some reliable data showing Firefox usage at 30%, 40% or even 50%, the PR folks would be pointing me to this rather than NetApplications.

So, is Mozilla's PR team underestimating the browser's market share by choosing to believe NetApplications?