It really depends on what you want to do. Opera is pretty much a one stop type source of all things internet, it can do mail, handle a plethora of news net protocols and is generally robust and portable to other platforms (if you have multiple computers you can synch your bookmarks and speed dial between computers). It comes loaded with featurability for both development tools and a number of common user tasks, is very secure and boasts some of the fastest page loads possible. Firefox is the other big name, it comes with fewer features but can be added to using a platform called GreaseMonkey. Due to the lack of preloaded features it is slightly faster loading than Opera (Not by much though, ~1500 ns faster on a modern processor) though addons and modules can bog it down. Once again, Firefox is among the more secure browsers however, it does attract attention from hackers simply because it is getting more use.
