Yslow FireFox plugin could give many useful recommendations for improving your site speed like gzip, aggregate js & css, enabled caching, CDN issues and more.

Download from here

https://addons.mozilla.org/en-US/firefox/addon/yslow/

This add-on requires installation of Firebug plugin as well.

After installation find icon of Yslow plugin on the bottom status bar – right icon on the image below