Internet Explorer has like 93% user base, so unless lower user base is better....
Ok.... So, let's say FireFox has a 5% user base. It then takes 3 days to fix a patch.
3 days x 18 = 54 days. 54 days / 7 days/wk = 7.71 weeks.
Assuming the breadth of market share that Microsoft has to cover grows proportionately to its user base, MS is actually beating FireFox, and that's assuming IE has 90% market share to FireFox's 5%.
In other words, MS has to make sure that IE works on a much broader range of systems than FF does.