Firefox 3 has broken new ground in browser usability. The address bar has taken on a life of its own. Going far beyond address-matching as you type, Firefox 3 also matches your entered URLs against keywords within the title or tags of the page. It sorts by frequency and recency, and tunes itself as you use it. I've found that it gets the right page or link for me just about every time.
The combination of the smart address bar and the new page-tagging feature for bookmarks can make finding pages you've visited incredibly simple. Bookmarks are now organised in a database, not in a flat file, and thus are easier to manage and search. Smart bookmark folders can be created to automatically arrange bookmarks meeting certain criteria based on tags and other information. All of these features are impressively handy no matter what I seem to be doing with the browser.
On platforms other than Windows, Firefox 3 has made a great effort to integrate better with the host OS. Firefox 2 on the Mac platform, for instance, always had the feel of a foreign app. It functioned well enough, but it didn't share the OS X look and feel. It does now, and it even supports OS X Widgets and Growl. On Linux, Firefox 3 uses the native GTK theme running on the system to provide a better visual fit.
As with Firefox 2, the customisable Search toolbar is right at home, offering any number of existing search engines from Google and Yahoo to Wikipedia, YouTube, and eBay.
As far as add-ons go, it might take a little while for all of your favourite accessories to come up to speed with Firefox 3, but I've had few problems in that area. In fact, Firefox 3 has led me to use some add-ons that I probably never would have discovered, all due to the Recommended page in the add-on manager.
Some of the most important add-ons for me, such as the simply indispensable Web Developer toolbar, have been Firefox 3 compatible almost since day one. Others will follow soon. Sadly, I can't seem to find the Abe Vigoda status add-on anymore. Surprising that Abe outlived it, I suppose.
The speed and resource requirements of Firefox 2 were a sore spot for many users. There were certainly instances where Firefox 2 behaved nicely, but those were overshadowed by the times when loading a page with certain embedded elements or other code would cause Firefox to crank up the CPU and start eating RAM like candy. Often, closing the offending page would reduce these symptoms, but sometimes quitting and restarting the browser proved the only solution. Firefox 3 hasn't been free of these episodes, but the frequency has been greatly reduced.
If the Web browser isn't the most important application ever developed, it might be the most personal. If you're working in a company that regrettably invested in Web-based applications that cannot function without using Internet Explorer, you have my sympathy. For those of you who have a choice, you no doubt want a browser that functions as an extension of yourself: customisable, quick, reliable, and stable. Firefox 3 meets all those criteria for me, and there's no looking back.