Since I happen to have the data handy, I thought I'd update the historical chart. This is chart represents the monthly trends as recorded by Net Applications over the last 56 months.
What's pretty clear from the chart is that the long-term trends are very linear.
Safari's growth, which is really just a proxy for Mac growth (Safari is not gaining any appreciable share on Windows) works out to about 7 points over these 56 months. Firefox's growth clocks in at about 20 points over the 56 months. And IE's loss has been a little more than 26 points in that timeframe.
What's really kind of sad is that while IE has lost 26 points of share since October of 2004, IE has actually grown its user base by about 100 million users.
So, for all the talk of browser competition, a clearly inferior browser has managed to add a huge number of new users over the last few years. There's the big advantage of being the default browser for virtually every new PC that ships.