In my opinion, Xenocode Browsers is the ultimat tool for checking your website in different browsers. Xenocode Browsers allow you to lauch IE6, IE7, IE8, Firefox 2, Firefox 3, Google Chrome and Opera directly from the web. The only weak point: The service isn't available for Macs and GNU/Linux powered PCs.
Today, I attempt to find out which one add-on can replace my skin of Firefox, and easy to install. "Personas" is a best choice for me when I attempted to install many skin add-ons. If you also want to make your Firefox to be beautified, you shall try to install it.