FF cannot reconnect after a network change even after all other softwares are seeing the new network.

Moving with my notebook computer from one WLAN to another (in another part of the building), either closed (thus in "sleep mode") or not closed, very often results in a loss of all connections. FF says that it cannot find the servers any more, while a direct "ping" or other softwares have no problem seeing and connecting to the servers.

I tried different proxy settings, but this did not change the situation. Normally the only "solution" is to close or "kill" the firefox, and after a new start also the FF finally "gets it" properly.