Two interesting extensions that add speech support to Firefox:

Foxyvoice

FoxyVoice is a Firefox extension that provides text-to-speech functionality using Microsoft Win32 Speech API. With FoxyVoice you can listen to the page being read; or browse on one page and listen to another page being read for higher degree of sensory overload. The soothing voices that comes with SAPI also makes FoxyVoice a competent virtual hypnotist.

Accessibar

Accessibar is a toolbar extension for the Mozilla browser which aims at providing various accessibility features for users who could benefit from them. These features primarily focus on the dynamic manipulation of the visual display of the web page in addition to the integration of a text to speech reader which can read out loud the browser's user interface as well as web page content.

As this extension uses the Java based FreeTTS engine, it works cross-platform. In addition, it offers a variety of other interesting features, such as the ability to select foreground/background colours, change text size and line spacing, and disable images.