Forex Currency Converter
This handy plugin does just one thing. Once installed, it'll turn any currency amounts listed on a Web page into hyperlinks. When the links are clicked, a small floating window appears that can convert the amount to any other currency based on current exchange rates, with data from iforex.com.
In other words, if you're browsing a European site and are thinking of ordering a product, simply clicking the price in euros will allow you to instantly and accurately see what it is in U.S. dollars.
A silly irritation is that the add-on doesn't detect the currency, so you'll need to select it from a dropdown list in the pop-up window. However, the pop-up remembers what you selected last time, so if you frequently have to convert Euros to US dollars, for example, there'll be little hassle.
Note: This link takes you to the add-on's page at Mozilla.org, where you can automatically install the file into your Firefox browser.