Somethimes I am wondering what is going on with my internet connection. Firefox is not loanding anything although I have a transfer rate of 7 kB/s (what is somehow fast here). But why? To track everything with programs like wireshark was too big for me figuring out, which program is using the bandwith.
But I finally found a simple program called nethogs in the package archive.
After installing the package nethogs
$ sudo apt-get install nethogs
you run the program from the terminal
$ sudo nethogs interface
wheras the interface is the network interface you want to monitor, such as eth0 (usually for the cable connection of your first network card), wlan0 (usually the first wlan interface) or in my case ppp0 as it is a mobile broadband connection. And you get a nice overview over the programs using the network interface.