One of the major performance related changes in Firefox 5 is an improvement in the way that keep-alive HTTP connections are re-used. Previously, there was a simple FIFO queue. So if Firefox ever tried to reuse a TCP connection it would simply use the connection that had been idle for the longest period of time.
However, not all connections are equal. Connections that have transmitted the most data are likely to be faster than those that have only received a small amount of data. This effect is caused by the congestion window mechanism in TCP.