The UX behind this isn't really obvious... 
There is multiple ways to improve that:
* Display an alert() when we add the app,
* Display app kind (hosted or packaged) in the search results,
* Use different colors for packaged and hosted apps,
* Put a `*` at end of app name and in beginning of this warning message,
* ...

Also, do you know why it doesn't work? It may require various tweaks, but packaged apps should work in Firefox.