Of the three big-name browsers, Firefox is the only one not offered by a company that has a vested interest in tracking everything you do. That's the reason why I started using Firefox long ago, and it keeps me coming back.
Firefox is at the forefront of so many browser improvements it's hard to catalog -- or even categorize -- them all. Significant interface improvements. Security. Stability. Performance. Phishing block. A developer ecosystem. At one point or another in its lifetime, Firefox has led the pack in all of those categories and more.
Hats off to the scrappy folks who knocked Microsoft off its dusty IE perch. If it weren't for Firefox, we might all still be running Internet Explorer 6. Sobering thought, that.