Firefox Nightly has introduced WebIDE. You have to switch it on to use it, but you will want to switch it on if you have any interest in creating web apps.
Considering how important web development is, it is surprising how difficult it is to work out what tools to use to develop a web app. Firefox has been improving its debugging tools for some time, but now it has introduced a full IDE that you can use to generate the code you test. This means that you never have to leave the browser to create, test and modify your app.
At the moment it is targeted at creating Firefox OS, Firefox Desktop and Firefox Android browsers. It is logical for Mozilla to start out targeting Firefox, but the intention is to extend the support to a range of mobile browsers including Chrome for Android and Safari on iOS. If the project gets to its target you will be able to use WebIDE to create a web app for the major mobile platforms as well as the desktop.
When you want to test you code you simply select a runtime. If you select Firefox OS then the IDE will either use a simulator or a connected Firefox OS device. You can then use the standard developer tools to test and debug the app. For example, as you move the cursor around the app's UI, the HTML that generated each element is displayed in exactly the same way as when you inspect a web page. You can also modify the CSS and see the effect at once. The idea is to create a rapid iteration environment and all you have to do is hit Ctrl/R to reload the app after making changes.
Mozilla has a video to show you it in action:
WebIDE is based on Mozilla's Firefox Remote Debugging Protocol. This is what is used to make remote connections to browsers running on mobile hardware connected via USB. As the protocol is open source, Mozilla encourages its re-use.
Of course once you get beyond the simple self-contained app, things become slightly more complicated. If you are building an app that needs a backend server, you are going to have to set things outside of WebIDE to make it all work. It seems strange that we are only just tackling the problem of providing adequate tools for the creation of web apps. It is arguable that even WebIDE isn't yet adequate for the job, but it is very much a step in the right direction.