Exch was a small utility written in XUL and JavaScript that could be installed as a Firefox extension or as a standalone desktop application. While it allowed the user to enter two currencies to convert from one to the other, it was dependent on a now defunct data source from Yahoo! Finance and is no longer maintained.
![](https://content.karlherrick.com/wp-content/uploads/2012/02/exch-extension-2005-2011-1.jpg)