It is simply 3 points:
1. Plugins: This is absolutely the best part of FireFox, it is a excellent job.
2. Overall tests is not in the last position, some important tests are No.1: Chrome and Firefox 3.5 Memory Usage, Big browser comparison test: Internet Explorer vs. Firefox, Opera, Safari and Chrome
3. Not only a browser, it is also a platform: Forget Facebook. The Web’s platform is Firefox