That article doesn't state that only 7% of Mac users are running Firefox. It says 7% of Firefox browsers are on Macs. With Macs currently accounting for about 10% market share of PCs in general, You could have 7 in 10 (70%) Mac users running Firefox and that still only account for 7% of Firefox's overall usage because the remaining 30% of Firefox users are spread over a much larger base of Windows users.
According to:
Browser market share
Firefox total market share is about 20%. And 7% of 20 is 1.4%.
So 1.4% of total computer users which = 14% of Mac users or 14 out of every 100. (Based on 1.4/10, the 10% share of Macs).
Firefox's total PC marketshare is 93%. And 93% of 20 is 18.6%.
So 18.6% of PCs 90% total market share = 16.74% or roughly 16/100 PC users use Firefox.
So in short for those who hate maths
is:
14% of all Mac users use Firefox.
And 16% of all PC users use Firefox.
So it's roughly even at the moment.
But It's still a lot more PC users cause they are much more numerous.