The good folks at the Mozilla Foundation have unveiled an amazing suite of Web-development tools. Wired's Webmonkey has a great summary:
The most popular request, and by far the coolest of the bunch, is the ability to do live edits in the text editor of your choice — effectively controlling Firefox with your editor. The video below shows an example of live editing via the popular Sublime Edit. This would essentially eliminate the need to jump from your editor to the browser, hit refresh, jump back to your editor, and so on. A dance that most of us are all too familiar with. Perhaps the best part, Rouget says this will work with the mobile version of Firefox as well.
Mozilla is also working on the opposite idea — authoring in the browser. That means putting an editor inside Firefox's Dev Tools suite. Thus far this idea is less fleshed out, but the possibilities include putting in something like jsFiddle or perhaps a more traditional file-based editor.
Other new tools include some catch up features that bring Firefox's Dev tools up to speed with what you'll find in WebKit browsers. Examples include a new network panel prototype and the ability to doc the tools to the right side of the screen — great for wide monitors (this is already available in Nightly). There's also a new "repaint" view that shows what gets repainted on the page, very useful if you're trying to improve performance. Rouget has also been working on a new, dark theme for the Firefox dev tools.