Tenon.io is a new platform to test your website against the most common accessibility and usability errors. This add-on allows you to quickly check the page that is loaded in your current tab against the Tenon test suite and get immediate results without having to write JS and validate the returned JSON code yourself. The results will be displayed in a new tab. To invoke the test, you can either use the toolbar button, or right-click your web page and choose the "Check page with Tenon" context menu item. This latter also works with the keyboard, so you don't even need to use a mouse to test your web page for accessibility. The web page must be on a publicly reachable URL.
Note that Tenon.io is in free beta until January 31, 2015, and will then cost a fee if you want to use advanced features. However, this add-on should continue to work without having to pay a fee.