ho trovato questo....

If you get a message like
-----8<------------------snip--------------8<--------------
Xlib: connection to ":0.0" refused by server
Xlib: XDM authorization key matches an existing client!
----->8------------------snap-------------->8--------------

you have to disable the X authorization with command
'xhost +' and startup firefox.
After the initial startup you can close your X session
again by 'xhost -' and you should be able to start firefox
from now on without problems.