YSlow (sometimes mistakenly written as "WhySlow") is a very handy utility from the Yahoo Developer team (http://developer.yahoo.com/yslow/) which allows you to profile the performance of your pages in FireFox. It requires FireBug to be installed first, and runs inside your FireBug panel.
One of the simple and very handy features that the Internet Explorer Fiddler Tool 2 (http://www.fiddlertool.com/fiddler/) doesn't currently have is the page size and page timing toolbar in the bottom right hand corner. I recall that Netscape used to have this about 15 years ago, and Safari and Opera current have this feature. The only reason I can see why IE and Firefox don't have this by default is that the masses would get unhappy when they realize just how slow their pages are currently loading!
From now on, this will be my first port of call when making speed and size comparisons between sites.