Yahoo! has just released a new Firebug plug-in called ySlow. I just finished installing it and playing around with it for a few minutes and it definitely looks like it could be one of those must have web developer tools. It provides the following:
The Performance Report card is pretty slick. It provides lots of tips on improving performance—such as ways to minimize HTTP requests, removing duplicate tags, adding expiration headers, etc.
The Stats View provides a nice run down of the byte size of each HTTP request and even includes the cookie sizes. You'll also see the impact browser caching has on the page—which is really nice.
I'd definitely recommend installing this and checking it out.