Now some would argue that Firefox deserves to be your browser for the great number of cool plugins that it offers. This story in Computerworld is typical of what the Web savvy are saying about Firefoxs plugin advantage. The Firefox website has this list of 1844 and counting add-ons.
Others will say that Firefox real advantage is that it supports W3C and other Web standards such as SVG, E4X, and CSS better than IE and most other browsers. And our recent tests of CSS and AJAX would bear that out vis a vis IE; but not Opera and Apples Safari which seem to match and sometimes better Firefox at least in some CSS tests such that one cannot unequivocally say Firefox is better than Safari and Opera on CSS tests.
But the most important ones are Web Developer and Firebug. These are debugger tools built right into Firefox that speed up development time so significantly. The screen shot below show how much information these debuggers present: