Incredibly, Firefox has been downloaded close to 50 million times. Its market share across all operating systems is close to 6% in the United States (see WebSideStory data) as of February 2005, and this share is growing steadily - although now at a decreasing rate. It may not be a huge player, but it has probably kicked Microsoft into releasing a 7.0 version of Internet Explorer, which is impressive in itself.
For sometime now, Firefox (previously Phoenix and Firebird) has been my browser of choice. I see no reason why that should not continue, even with advances in Internet Explorer and Opera on the horizon.