Text to Voice is a Firefox plug-in that adds text to speech to any web page. After you install the add-on, you will see a small speaker icon in the lower right corner of the Firefox window. To use the extension, select some text on a web page and click once on the speaker icon. You will then hear the selected text.
This extension currently supports two voices (Male and Female). Of the two, I found the female voice to be the best in terms of clarity and pacing. The preferences can be accessed by right-clicking on the speaker icon.
You can also use the extension by selecting some text on the page, right-clicking on the selection, and choosing Speak It!. This should bring up a small window (your popup blocker might need to be disabled). One of the options on the popup window will be a link you can click on to download an MP3 version of the selected text.