Add-ons I use for SEO and accessibility testing. I grouped these 2 topics together since they use some of the same tools and have similar goals.
The Web Developer extension adds various web developer tools to the browser.
Display the Google PageRank, Alexa rank, Compete ranking and SEOmoz Linkscape mozRank anywhere in your browser, along with fast keyword density analyser, keyword/nofollow highlighting, backward/related links, canonical links, WHOIS, robots and more.
Support web developers in testing web resources for accessibility features.
Fangs renders a text version of a web page similar to how a screen reader would read it. The ambition is to help developers understand how an assistive device would present a website and thereby increase chances of finding accessibility issues early.