Firefox could really use an extension that tells you if the web page you're visiting is valid (X)HTML. Navigating to a page, and then going to W3C to validate the link is ok, but it's sort of slow. Not to mention that it relies on the W3C server being up. If Firefox had some extension that sat in the task bar and showed a green light for a page that has a DOCTYPE and is compliant, or a red light if it's not compliant would be really handy. Ultimately the best would be if Google started ranking pages higher that were valid markup.