I got my firmware from this site:
http://www.redmondpie.com/download-iphone-os-3.1.3-firmware-9140418/
Already download it and tested it. Works fine and my iphone is much faster now.
iPhone 3G. The latest update 4.x has made my iPhone very sluggish. I found a link to previous versions of firmware, specifically 3.1.3 with simple instructions, and promptly lost it. Can someone supply a link?
Thanks large.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic