Hi,
I had this problem too. This was the only thing that worked for me:
http://hilfe.o2online.de/t5/Router-Software-Internet/Ständige-Verbindungsabbrüche-mit-O2-BOX-6431-WLAN-Problem/m-p/801198/highlight/true#M57367
You have to go to the configuration site of your o2-router. Than in the firefox menu go to “Ansicht” - “Webseiten-Stil” and choose “kein Stil” (sorry, don’t know how this is named in english). There you can change the WiFi mode to 20 Mhz. This should solve your problems.