Fangs renders a text version of a web page similar to how a screen reader would read it. The ambition is to help developers understand how an assistive device would present a website and thereby increase chances of finding accessibility issues early.
Here is a little more about the add-on from the developers:
Renders a text version of a web page similar to screen reader output. Helps developers find accessibility issues in web pages and managers to understand how their website may appear to a person using an assistive device. Fangs is used by web developers, teachers and Quality assurance staff to make websites usable by as many as possible.
Want to learn more about Fangs? You can pick up the screen reader emulator from Firefox’s Add-on database.