Well I've tested your script with an old backup of Zope 2.3.2 on an Win NT SP 5 machiene. There are no problems. The script runs in one window and in the other I can surf the management screen. My Solaris can't do that.
Just for clarification: On Linux I need _two_ browser windows holding a connection to zope. I know nothing of how zope threads what, but together with that "stop" trick it looks like I have seen two different thread limitations. One is the zope-configurable limitation and another one seems to be related to the number of connections zope can serve under this circumstances. What I haven't done is ruling out that perhaps the browser might be the culprit - I doubt that, but I can't rule it out atm. Will test it later. cheers, oliver