Hammerhead – measuring the load time of web pages

Hammerhead adds a tab to Firebug for measuring the load time of web pages. Hammerhead adds an icon to the Firefox statusbar. Next to the icon is shown the current page’s load time. Right-clicking on the icon displays a menu from which you can quickly clear both caches (disk & memory), or just the memory cache.

and much more

http://stevesouders.com/hammerhead/