I have been looking at the server traffic logs for this site over the last few months, and noticed some interesting trends happening in the area of web browser usage. It would appear from the logs for this site (admittedly a narrow sample), that Internet Explorer usage is declining quite steadily, Firefox usage in increasing very slightly, while non-Firefox Mozilla usage has begun to increase quite rapidly from 3.4% to 13.6% in the space of seven months. In the same period, IE usage has decreased from 66.1% to 53% and shows no signs of slowing this descent.
What is evident from these statistics is that the days of designing websites for IE initially, and then hoping that your site would work on the other browsers, are now truly gone.
Updated 2021 : note that the above post is out-of-date, given this post was originally published in 2006, but is left here for archival purposes.