I use the YSlow extension for Firefox, but prefer the Page Speed extension for Firefox.
Page Speed will give lots of feedback, detailing how you can improve response times. It also provides practical assistance. For example, offering you optimised image and css minimised versions.
To use Page Speed, you need the Firebug Firefox extension already installed. There is also a Page Speed Chrome plug-in available, but I haven't used it.
Alternatively, there is an version of Page Speed that you can run online at:
Page Speed Online