Cut and pasted in its entirety from another forum I frequent. A lot of you probably know about it, but for those who don't, here you go.
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Here's something for broadband people that will really speed Firefox up:
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
<<snip>>
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"
<<more snip>>
3. Lastly right-click anywhere and select New-> Integer. Name it "nglayout.initialpaint.delay" and set its value to "0". This value is the amount of time the browser waits before it acts on information it recieves.
<<parsnip, read below>>
If you're using a broadband connection you'll load pages 2-3 times faster now.