WebAIM recently released the WAVE Toolbar for Firefox, which augments their free online accessbility evaluation service, known as WAVE. WAVE is currently in version 4, much like Cynthia Says but with a focus on developer friendliness by overlaying results visually above the page you are testing. I think WAVE is a pretty cool idea. The fact of the matter that no automated tool can ever tell whether a website is truly accessible means it always boils down to human assessment in the end. So a visual tool is much welcomed as the explanations you get from Cynthia are quite cryptic at times.

For more power in your toolbar I recommend trying out the Firefox Accessibility Extension. This one is primarily an assistive tool for people with disabilities browsing the Web but also doubles up as a great accessibility evaluation tool for developers.

Operator is a Microformats detection toolbar for Firefox 2. The toolbar makes developing and using microformats much easier and more intuitive. You could export hCard data on any Microformatted page to standard vCard format by selecting it in the toolbar, no copy & paste necessary. And hCalendars could be exported in a similar fashion to your favourite calendar package. Here's a good introductory piece on Operator by Alex Faaborg, a User Experience Designer working on Firefox 3.Microformats are growning in prominence and the Mozilla team is hot on it's tail. Most of the functionality you see in Operator now is likely to be part of the Firefox 3 core!