One of the most useful Firefox extensions I have installed must be ViewMyCurrency, which helps me to convert currencies inside the browser. Basically you define which currency you want all prices to be converted into and then many major world currencies will automatically be converted and displayed along with your own currency inside any web page.
If you regularly browse sites with prices not in your preferred currency you will greatly benefit from this extension. I have been using it for several months now both at work and and at home, and I can't imagine surfing without it. Highly recommended, and of course free!