To inspect for HTML validation issues, right-click a webpage and choose HTML 5 Validation from context menu.
This will open a separate window with HTML validation information of the web page. In case you are wondering why anyone would use HTML 5 Validation instead of Firebug? The answer is simple. HTML 5 Validation is specifically designed to provide in-depth information about HTML related issues on a webpage (including the under development HTML 5), whereas Firebug despite acting as a comprehensive toolkit for developers can appear quite convoluted. This extension on the contrary provides straightforward and easy to debug information for the HTML5 format of a page.
HTML 5 Validator extension works with Firefox 3.5 – 3.6.*.
HTML 5 Validatior Extension for Firefox