Nowadays, if you want to find something in the internet you have to use browser.
And when it comes to the question “which?” you will have many ideas and criteria
which one to prefer.
One might choose the default one available in the initial system configuration, for example:

• explorer in windows
• safari on mac
• firefox in some gnome-based linux distributions, or konqueror in kde-based

Other would prefer ones available with development and debugging tools.
Third group of the people will look into more secure and safe means of browsing: session manager, advanced history tracking, and etc.
For the fourth, the most important would be to have much theme plugins, to make interface visuals styled in a different way.

As you can see, there are many requirements from various user groups.
But what market has them to offer?

According to review here, browsers were ranked in the following way:

1. Firefox
2. Chrome
3. Explorer
4. Opera
5. Safari

Mainly, its shows ergonomics and user-friendliness of the given browsers, and indeed Firefox is the best one (my personal opinion) in this list.
However, if you look into more technical details, then the picture will be different.
As for support of modern CSS2 and 3 is done fairly well by most of the browsers, it is still retarded in all versions of the Internet Explorer, not to mention support of html5.
(more details here)

So as a conclusion I would say, that its important to know your needs when it comes to choosing the browser, because each of them have drawbacks and benefits.