There are quite a few nice web development tools out there. I'd like to point out two Firefox extensions that IMHO stands out:

• The Web Developer toolbar by Chris Pederick
• Firebug

I don't think you'll need both of these extensions, so try them out and pick the one you like best. Firebug just hit version one dot oh and I installed it for the first time two hours ago. So far I'm really impressed!