After alot of thinking and testing weve come to the conclusion that its the http timeout value in about:config (which defaults to 300). Its in conflict with the timeout value of our firewall for the HTTP-Protocol.
The setting in the firewall is 60 seconds which means that if a http-connection lasts more then 60 seconds in firefox (it will last 300 seconds) the firewall expects a TCP-SYN packet to start a new http-connection, firefox however will NOT start a new connection since it thinks it is still connected. The firewall will look at the next packet and think its out-of-state, which makes firefox "load" the page until it reaches its timeout value (300).
The solution to this specific problem is to set the http timeout value in firefox lower then the one in the firewall to make firefox initiate a new conneciton before the firewalls timeout ends.
