LB::server should just log the most recently selected pool name:
http://devcentral.f5.com/wiki/iRules.lb__server.ashx
LB::server returns a Tcl list with pool, node addr and port. If no server was selected yet or all servers are down, returns default pool name only.
Heinen, what actually breaks when you test with Firefox? Can you use HttpFox or Fiddler to record the session and see more exactly what's failing?
Do you have a OneConnect profile enabled on the virtual server? This is necessary for evaluating each HTTP request on a keepalive connection. See this article for details:
http://devcentral.f5.com/wiki/AdvDesignConfig.ONECONNECT.ashx
Aaron