In bug 697383 a previous iteration of navigator.mozApps was implemented, but that was only packaged in XUL Fennec and doesn't match the current webapps work.
Some of the code is still on trunk in dom/base/Webapps.* but currently unpackaged.
Fabrice said he is working on this on bug 720415. We need to determine if there's more to it than bug 720415 or if this one will end up being a dupe of that. In any case I'm filing this one to make sure this gets tracked
Fixed by bug 720415 + Bug 697006 part 1. Thanks Fabrice!
Verified that this feature has landed.