According to the latest web metrics, IEs share of internet browser programs using the web has dipped to 89.3% in February, down below 90 for the first time.
The growth of Firefox is now slowing. Until someone starts making it a default somewhere on new computers it is not likely we'll see any more huge growth numbers for Firefox, thouhg the gradual bleed away from IE will continue Microsoft updates the browser.
Depending on when that is, it wouldn't be surprising to see more Windows 2000 users (and older versions) move to Firefox, especially in corporate environments where a mass rollout is easier. Not that there's a cost to using one or the other, but as Firefox continues to get updates, and as most people are satisfied with their OS, it is expected that many corporations will look more favorably upon the switch.
That's assuming that there aren't more and more attacks on Firefox. Although updates are occurring quickly, quick patches also leave open the chance that the fix will break something. Or that the fix opens some other hole. Firefox has the advantage now, but they need to keep it by performing well and not making mistakes. In the meantime corporate web serices will continue to be developed towards the IE feature set.