From the author, Peter Krantz:
In 2005 I wanted to learn more about how screen readers worked. I discovered that the most commonly used tools were too expensive for web developers to buy. Alas, many of my web developers friends knew little about how their websites appeared to a user using a screen reader. By developing Fangs for Firefox in my spare time I hope to make it easier for people to understand how a web page may appear in a screen reader and make the web a better place for everyone.
Click this link to learn more or download the Fangs Screen Reader Emulator.