JSON Editor seems like a nice fit for the manifest editor. The right panel is easily reusable. : http://jsoneditoronline.org/
Try builds to test out a basic prototype: https://tbpl.mozilla.org/?tree=Try&rev=856e1286915c Current functionality: * Displays manifest in editor * For packaged apps, the update button will save the manifest and re-install
We've decided to extend VariablesView instead.