this problem seems to occur with versions from about 3.5 onwards. In my experience, it is only a problem with Vista and you need to reboot the machine to get a new connection. Re-starting the browser doesn't do it. I also find that if I launch Firefox and IE at the same time BOTH browsers loose the connection to the internet. Thunderbird mail tool still fetches mail so the internet is still alive, just the browsers fail to connect to it.
Rans6Andrew