Although I prefer IE7's native speed and feel for day-to-day browsing chores, there's no doubt that Firefox is my primary web development IDE.
WDE has lots of useful features, but the most wonderful thing about WDE is that everything is editable in real-time. Edit the CSS, edit the HTML, and watch your changes take effect as you type. It's like magic.
I can't imagine debugging a web app without these two essential tools, which transform Firefox into the Visual Studio of browsers. It'd be akin to writing a .NET application in Notepad. In fact, I feel so strongly about the utility of these extensions that I'm stopping to donate $10 to each project, right now. This is part of my ongoing monthly contribution to small software vendors.
If you haven't used these tools yet, I envy the experience you're about to have. Download them both and start experimenting. If you have used Firebug and WDE, I encourage you to share your favorite tips in the comments.