Analytics for me says, for the last month:
Firefox 8.0: 2.68% Mac
Firefox 7.0.1: 9.53% Mac
Firefox 3.6.24: 13.47% Mac
So it's definitely appreciably lower for 8 than for 3, but the closeness between 7 and 3 makes me wonder whether it's actually an effect.
I'd be curious to see how many of the 3.6 laggards really are on old macs and are simply unable to upgrade until they budget in some new hardware.