Firefox is not the best browser. I think Opera is a technically superior solution to render HTML and XML. However, Firefox is easy to customize and that is, more than anything else, its main strength over any other browsers. It might have taken years, but the Netscape dream of an all-powerful web platform has come true with Firefox.
One feature I really like with Firefox is the integrated search engine box. However, I think that the defaults are disappointing. The Wikipedia search engine should come by default. And if you are a scientist, you need an integrated Google Scholar. It is too bad that adding new search engines cannot be automated easily.