MTU adjustment needed on Orange wifi

I'm on the road. On the hotel, there's wifi, provided by Orange (France Télécom). The connection has been strange: Most of the time, I couldn't connect to Facebook and MSN Messenger (XMPP/Jabber connections worked fine, though); SSH connections were also unstable. Finally found out that it helped changing my network settings such that the MTU parameter was decreased to 1200.