A friend & I are both using build 4.8.1229 with similar setups - Vista 32bit (he has Home Premium and myself Business Edition) + ZoneAlarm freeware firewall, no other antivirus etc tools - except he sticks with IE while I use Firefox 3.0.1 (he has never installed Firefox). My friend has no problems, whereas I have had the same experience in this thread of all internet (and network!) access halting after having opened about 20 webpages (rough estimate only), whether or not I close & re-open the browser every now and then, or use IE instead, and using Flashget download manager also seems to contribute to the 'total' events. Firefox then gets 'stuck' in that it won't unload, and the only way around it for me has been to reboot (which itself then takes such a long time I sometimes have to do a reset). I think it is not specifically because of ZoneAlarm, as I had the same problem with current version of Outpost Pro.
The SOLUTION (for me) ... I de-activated the WebShield component of Avast!. Immediately, all is now 100% OK.
My current thinking is I am happy with Avast! and want to stick with it, though having to do without WebShield worries me somewhat, so hoping for a fix soonish (whether from Alwil, Microsoft, ZoneAlarm or whoever). When I have WebShield on the problems for me are so severe that to re-enable it in the meantime is not tenable for me.