The default install of Mozilla Firefox does not take advantage of multiple simultaneous connections to web servers when downloading files and images. Please note the following tweak is only useful if you are on a high speed connection (not dial up).
The following is quoted from the Free Republic website on how to change this:
1. Type "about:config" into the address bar and hit return. Scroll down and look for the following entries:
network.http.pipelining
network.http.proxy.pipelining
network.http.pipelining.maxrequests
Normally the browser will make one request to a web page at a time. When you enable pipelining it will make several at once, which really speeds up page loading.
2. Alter the entries as follows:
Set "network.http.pipelining" to "true"
Set "network.http.proxy.pipelining" to "true"
Set "network.http.pipelining.maxrequests" to some number like 30. This means it will make 30 requests at once.