For some medium-sized, low-tech site:
IE6 49.8%, IE7 34%, other IE: 0.7%, total IE 84.5%
FF 2.0.x.x 7.5%[1], other FF 1.9%, total FF 9.4%
Others 6.1%[2]
1. FF 2.0.x.x number has been arrived by adding the percentages of the seperate versions, so there's some rounding error there. Other FF has been arrived at by subtracting FF 2.0.x.x from total FF (which is accurate).
2. Similarly others has been derived by subtracting the totals for IE and FF from 100.
The overall market share of IE6 is 48%
IE7 is 31%
Firefox across all versions about 14%
That will give you a better idea. Obviously the number is highly different for technology related sites. Example: I remember a popular tech blog mentioning that their firefox users were over 50%. So it really depends on your audience as well.
I have fairly accurate numbers for reddit audience (had a page with 100% traffic referenced by programming.reddit.com, approx 600 visits): Firefox (68%), Safari (8%), IE6 (5%), IE7(6%), Opera (5%) and a few marginal ones.
IE6 is the worst thing about creating sites right now.