Fangs is a Firefox add-on that "creates a textual representation of a web page similar to how the page would be read by a modern screen reader".

This is useful for two reasons:

• It's free, whereas JAWS is not.
• It shows you a block of text, rather than actually reading the text aloud. Most of us will find this more efficient.

Be sure to read the Fangs FAQ if you get a blank page (I thought it was broken, but it's easily fixed).