You're likely testing a feature of FireFox without realizing it.  If the
URLs that you're loading simultaneously match and are in the same
browser process, it will wait for the first to load before running the
second.  See what happens when you do the same experiment in two
different browsers, or with a browser lacking that feature.  You'll find
your flock example is working fine, and you can verify that FireFox
hadn't even sent the request until after the first page loaded by
watching the traffic in ethereal.