Earlier quoted context omitted.
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.
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.