The Mozilla/Firefox Accessibility Extension makes it easier for people with disabilities to view and navigate web content. A toolbar provides easy access to navigation, styling, and keyboard enhancement functionality. Developers can use the extension to check their structural markup from the browser window to verify that it matches the page content. The Accessibility Extension helps authors to meet these kinds of accessibility practices that are so important for the browsing experience of all users and vital to those with special needs.

I find it more advanced compared to the WAI toolbar by the Accessible Information Solutions, but especially more easy to use since it comes with a toolbar and a system menu. Mozilla Accessibility Extension will help developers too much to make their website more accessible.