Do you use Firefox 3.5? You and along with most of the web, it seems.
According to statistics provided by TheNextWeb and StatCounter Global Statistics, the world's most popular browser is Firefox 3.5.
Of course, Firefox's lead is only when counting each discrete browser version as a separate entity. With all versions of Internet Explorer moved under the same category, Microsoft is still the leader.
For Mozilla supporters, however, this is still a great milestone.
Altered image source: Download Squad.