For some odd reason, the Firefox Add-ons page for the HTML Validator by Marc Gueury mistakenly states that this plugin is not available for Mac OS X. However, on the developer’s website, there are several Mac versions including PPC and Intel binaries.
This is an excellent little extension that validates the HTML of any page you arrive at. Double clicking the icon in the status bar gives you verbose results of the page you’re looking at. It includes the HTMLTidy validator as well as an SGML parser. Very handy.