Since it's first version, the switch from IE 6 was the easier thing ever. With tabbed browsing and popup blocking IE was way behind. As yeas went by, I was falling in love with Firefox. It's ability to allow an extension to be installed, you can customize the browser to your own need. Extensions like Firebug, Web Developer Toolbar, NoScript, IE Tab (for the purist :)), and many many more from programming tools, FTPs, uploaders, downloaders that will make your life easier in every aspect. Version 3 it's even faster, more secure and with tons of new features. It was release on June 17,2008. among the new features are the new password manager, One Click bookmarking, Smart Locations Bar among others. One of things that I liked from the very beginning of Firefox, was its compliance with the CSS standard. I designed for firefox and then use some hacks to make the website work with IE :). And it's more secure also...
It's a must have for every web developer and everyone who cares about web freedom and security