Update: This is an old post and the information is a little out of date. Internet Explorer now comes with a very decent set of developer tools.
Firefox is very likely the browser of choice for many web developers, and with one very good reason: Firebug, which is, IMHO, hands-down the most important web development tool around. Seriously, if you haven’t tried Firebug, you’re probably wasting a lot of development time.
It’s so good, Yahoo! have created a full-time position just to develop it. I saw a little preview during Nate Koechley’s talk at @media 2007; they’ve built a plug-in architecture, of which the first is probably going to be a page load analyser. It’s an IDE in itself — and it’s a free plugin!
Now, my personal opinion of Internet Explorer aside, I do appreciate the effort their team is making to engage the development community. In a new blog post they’ve listed a series of development tools and plugins for developers, so if you’re in the unfortunate position of having to develop in IE (or, perhaps, you are just a bit weird and choose to do so), you can experience a small sample of the Firebug goodness. Doesn’t beat the real thing, though.