I had the some problem, but the cause was different. I opened Firefox using unity dash and then locked it to the launcher, and run the following:
As you can see, and I don't know why, Unity dash was suggesting my desktop icon instead of global firefox.desktop. This desktop icon was working properly, but it seems that it cannot work properly on the launcher.
brunojcm@familia-desktop:~$ gsettings get com.canonical.Unity.Launcher favorites['nautilus-home.desktop', 'chromium-browser.desktop', 'gcalctool.desktop', 'gedit.desktop', 'gnome-terminal.desktop', 'ubuntuone-installer.desktop', 'remmina.desktop', '/home/brunojcm/Desktop/firefox.desktop']
I did the following to fix:
I also re-added Firefox to my launcher.
brunojcm@familia-desktop:~$ rm /home/brunojcm/Desktop/firefox.desktop