Nothing can beat Firefox 4,its fast safe and reliable.Nearest competitor Google chrome for sure.
Whats Great in Firefox
1)Beautiful Themes support.
2)Maximum speed(I use dial-up and using latest chrome,firefox,opera,chrome's speed almost same but have no app like "AD-Block Plus" which blocks ADs I don't want to see and load the page faster,it is supported by Firefox.IE-worst browser I have ever used)
3)It is Best up to bottom
So, Firefox all the way! Some marvelous extensions (especially the web developer one).