Since Chrome doesn't have real AdBlocker, it is logical that stat counter will show its number accurate, while Firefox share will be underestimated due to a large amount of users blocking StatCounter and similar tracking services.
I'd love to see statistics on how many people use AdBlockers and/or Ghostery vs the wider audience that would be picked up by StatCounter and friends. I would be surprised if they made a large dent in the statistics though.
We can only speculate on that number. But point is whatever percent that is, they are likely Firefox users, since Chrome doesn't do real blocking of those sites.
If the number of privacy concerned people is only 5% (a guess as good as any other guess), add that number to Firefox percentage, and the picture looks quire different.
