This plug-in allows you to validate HTML and CSS of the current webpage on-the-fly using the W3C validation engine and the Firefox plugin called Ubiquity. You can download Ubiquity here. Once installed, you can simply goto command mode (alt-space by default) and type either: 'validate-html' or 'validate-css'. It will tell you if the webpage you are on is either valid or not. You can also click on the "valid" or "invalid" link to view any errors on the page.
Screenshot of ubiquity-validate in action