Webaim, the accessibility people, have a review, by Jennifer Dance, of Chris Perderick's 'I can't remember what life was like before' Web Developer extension for Firefox. This is easily my all time extension for any programme but can be a bit daunting for some. Jennifer’s article describes all the menu items that are useful for helping to ensure your site is accessible.
http://www.webaim.org/techniques/evaluating/devtoolbar/devtool