Mozilla Firefox, formerly Firebird, formerly Phoenix, hopefully to not change names again, is now up to version 0.8. Touted as a technology preview, it is a very capable, lightweight browser, and currently the only serious competition to the Opera browser in terms of features and security. It is also, luckily, an Open Source project - basically making it free.
Released on February 9th, 2004, Firefox 0.8 includes all the features you would expect from a modern browser, including excellent CSS support, tabbed browsing, find-as-you-type (which allows you to just type keywords when viewing a page and matching ones will be highlighted) and popup blocker as well as being highly configurable. The vast array of extensions available means that the program itself is a small download, but that features can be added at will (and some of them are truly inspired - more info at the bottom of this article).
Firefox is highly customisable, and provides several options for themes, allowing you to control the look of the browser to a degree. However, the focus is on speed. It is very quick to load up and very quick to use on the web. It also has a very simple interface, making it a real pleasure to use. The focus is entirely on the browsing, with little to distract you.
If you are used to Internet Explorer, you may be put off the idea of trying a new browser. Many are used to using IE, and despite its many faults and poor security continue to believe it is better than other browsers. Firefox, despite it's lightweight look and feel, is definitely a heavyweight contender to Internet Explorer.
Firefox (and Mozilla) have a massive number of downloadable, free, extensions to the browsers, providing all kinds of functionality. If you want to make the most of the browser, I would highly recommend visiting Firefox's extension index and checking out what is available. A few of my personal favourites are listed below:
Mouse gestures are truly innovative. They allow you to perform simple, common tasks with just a subtle flick of the hand. They do take a few days to get used to, but once you are familiar with them, you will be unable to live without them. Although the Mouse Gestures extension for Firefox is very capable, I would highly recommend instead installing StrokeIt, which provides customisable mouse gesture functionality in all programs. However, if you only want gestures in Firefox, this extension is essential.
Another invaluable tool, the EditCSS extension allows you to edit the CSS of a page on the fly, and have the results of all edits displayed as you work. If you work with CSS-based layouts, this really does save huge amounts of time.
StumbleUpon is a very entertaining addition to Mozilla, and also available for users of Internet Explorer. Basically, it adds a toolbar to the top of the browser, with several buttons. Clicking "Stumble" will take you to a random site. If you like it, you click "I Like It". If not, you click "Not For Me". If you click "Not For Me", it will take you to another site. The more you rate, the more accurately the toolbar will take you to pages you like. It is a great tool for finding rare gems on the web - and there is a fairly active community to boot.