Could it be that the functionality is still disabled and that's what the problem was?
Searching here this afternoon to try to find out why I couldn't produce the same results that you did with FF.
FWIW, a friend of mine running Zorin LTS installed Alacarte and created a desktop file as well.
He explained that, . In Debian I was not able to follow the path to the executable using browse until I had added the icon in properties.