Going through this thread:

proxy ARP WiFi bridge Raspberry

Ingo´s solution is working fine for me but I´m looking for an extra... With this WiFi bridge I want to use an add USB-Ethernet adapter (with fixed IP address). Both bridge and adapter work separately, but if I plug the cable in the new adapter, I lose internet connection in my bridge. The strange thing is that I´m able to go until the router, but I can not go outside. And as I told before without any cable plugged in everything is working well.

Does anyone know how to avoid this problem?