Help: No internet connection

[simon.schneebeli at okko.org]

Hello all,

My problem is definitively not linked to the fedora-webpage being  
down. Meanwhile I made some progress:

At my brothers place I managed to connect to the internet with no  
problem. All programmes worked, so I could add all the additional  
programs I needed and install the latest updates.

Now, back home, my problem is the following:
- DOES NOT WORK: Firefox, Thunderbird, Evolution (error connecting to  
the server)
- DOES WORK: Opera, Sype, Software update, Ping.

I guess that this is about the strangest problem that I have ever had  
since I use Linux. Help would me most welcome.

Simon