I’ve just come across a great add-on for Firefox that tells you what other prices are available for the product you are looking for without having to check a price comparison site.  Just underneath your bookmarks bar, you get tabs that appear telling you which retailer has the best price and what the prices are for other retailers.  You can go directly to any retailer from there.  It covers over 50 retailers in both the US and the UK. 

Simple and very effective.  Take a look and install it from here.