Weblog Of Zen and Computing reports that you may be able to speed up Firefox by clearing out its Download history. By default, the 'fox keeps a record of every single file you've downloaded - whether it's still on your hard drive or not - in its Downloads dialog, unless you clear it. To do so, from the dialog, hit the "Clean up" button. I don't know how much this list actually affects performance, but it's a good way to make sure that your downloads don't clutter anything.
Speed Up Firefox by Cleaning Out Your Downloads [Lifehacker-Of Zen and Computing]