According to one web-stat site, Firefox 3.5 has surpassed Microsoft Internet Explorer (IE) 7 and IE 8 as the most widely-used browser by version. However, Firefox use as a whole still has a long way to go versus IE use as a whole. So why all the fuss?
Some web apps and such have long been developed and maintained for IE exclusively. However, supporting IE at this point really means supporting three browsers: IE 8, IE 7, and IE 6. They are different enough to need individual tweaking and testing. If Firefox 3.5 has surpassed all three, even briefly, it has become very hard to argue that anyone involved in web development in any capacity can reasonably ignore Firefox any longer.