what happens if you just go to a console and type
firefox
Does it still work?
Now show the contents of the Xfce .desktop link you used.
If simply running firefox from the console doesn't work then it could be that the symlink is broken.
should show you using /usr/bin/firefox, which if you do a ls -l on it you should see it points to /usr/lib/firefox/firefox
If there is nothing wrong with the link or the desktop shortcut you used then you could replace the symlink at /usr/bin with a short script that runs firefox like you did from the command line
Code:
#!/bin/sh
cd /usr/lib/firefox
./firefox