I still prefer Opera, however some recent extensions to Firefox, make it a must have tool.

Get Firefox, install it. 
Run FireTune to tweak and optimize it

Essential extensions for webbing and developing

Web development

• Firebug (awsome for web development)
• Web Developer (even more tools at your fingertips for development)
• Header Spy easily look at the headers
• Lori (How long does a request take)
• XML Developer toolbar

Webbing/Surfing

• Fasterfox (Dramatic speed increases)
• NoScript (Selectivly block scripts on web pages)
• AdBlock Plus (Get rid of banners and ads) 
• AdBlock Updater (Use with AdBlock, loads new lists of stuff to block)
• Flashblock (blocks flash)
• Server Spy (What software is the web server running)
• GreaseMonkey (Allows you to customize web pages after they load)UrlParams (Shows all Get&Post parameters for current page)
• Extended Cookie Manager (Get a handle of the cookies) or use
• Cookie Safe (to manage your cookies)
• Locationbar (cleans up the url, decodes it, allows clicking on segments)
• ShowIP (shows the ip addy of the current page in the status bar)
• externalIP (Shows your external ip address in the title bar)