According to XiTi, Firefox has an aggregate usage rate of 24.1% in Europe. Some countries in Europe (e.g., Germany) have usage rates over 35%. In North America, the aggregate rate is 15% while Oceania (dominated by Australia & New Zealand) has an aggregate rate of ~25%.
Web masters need to sit up and take notice that writing for IE only is a good way to potentially disenfranchise up to 40% of their audience. We need web sites to rigorously adhere to W3C standards and for IE to come into compliance with W3C.
Like with Office formats, Microsoft needs to operate to international standards.