Screencast: [http://people.mozilla.org/~prouget/webide.webm http://people.mozilla.org/~prouget/webide.webm]
WebIDE will replace the [https://developer.mozilla.org/en-US/Firefox_OS/Using_the_App_Manager App Manager]. As for today, WebIDE only supports Firefox OS Apps (hence the name: App Manager).
Status: In Firefox Nightly. Set `devtools.webide.enabled` to `true`.