I've found a great Firefox extension that does this and a few other cool things.
It's called Tweak Network, and one can customize how it works. Anyway, it might be faster than doing about:config, although that works just fine:
Here's a link to it:
https://addons.mozilla.org/en-US/firefox/addon/327I do notice changes in performance, but I'm not sure that the speed has been a big issue, but I have a pretty nice connection as it is. Mostly, it's nice to be able to download more than a few things at the same time, and this extension makes that possible.