Seen this on Sean Corfields site, It's a plug-in developed by Yahoo, that adds extra functionality to fire-bug to allow you to track performance issues.
Get his Blog post on it here http://corfield.org/blog/index.cfm/do/blog.entry/entry/Improving_site_performance_with_YSlow
Get the FF update here here. https://addons.mozilla.org/en-US/firefox/addon/5369 and get the
Documentation for it here http://developer.yahoo.com/yslow/