Stay away from this tweak
These settings violate the HTTP protocol, and give you a speed boost by flooding the web server with 20-something connections for every single image and page request. There's a reason that they're not the default.
These settings will not only cause many web servers to have problems, but they can also make your web browser be mistaken for a flood attack, which will make the server add your IP to an "ignore" list. Stay away from these "optimized" settings, unless you know exactly what they do and how they work.
After seeing at least a couple dozen blog posts all referencing these changes to "speed up Firefox", I thought it would be worth a little explanation.
Yes, enabling HTTP pipelining can dramatically improve networking performance. The downside, and the reason it's not enabled by default, is that it can prevent Web pages from displaying correctly. If you've enabled this, and you find pages that aren't displaying correctly, please don't blame Firefox or the Web developer. It's probably the fact that you enabled an "unsupported" feature which is incompatible with some Web servers and proxy servers.
The second change, setting the initial paint delay at zero, may get you some content on the screen faster, but it's worth noting that it will dramatically slow down the time it takes the entire page to display. Here's what's going on. Gecko, Firefox's rendering engine, is trying to optimize between the cost of waiting for a bit more data versus doing more painting and reflows as new data comes in. Waiting a bit longer before it starts painting the page gives Gecko a chance to receive more content before chewing up CPU cycles to render and reflow the document. If you drop this value down to zero or near zero, that means you'll see the page start displaying a bit earlier, but not having received much data in that short interval, you'll have a lot more paint and reflow cycles to complete rendering of the page.
This one probably comes down to a combination of bandwidth, CPU speed, and personal preference. If it works for you, and you don't mind the side-effects, then great. Just note that what works for one person/system, may not work for another.
Yes, there are tuning change you can make (even at compile time, see Moox' optimized builds) that will dramatically alter the performance characteristics of Firefox. Feel free to experiment, but remember that most of the defaults are defaults for a reason. If your browser starts misbehaving or web sites look broken, it might be worth going back to default settings.
"Enabling pipelining in Firefox can speed up complex page retrievals, as you note, but it can also break Flash. This is a Macromedia thing not a Firefox thing but thats why the app defaults to pipelining disabled."