The Total Validator Firefox extension provides one-click validation. Once installed all you do is browse to the page you want to validate then click on the ‘TV’ icon in the browser status bar.
All of the options available with the online service are available with the extension through a series of tabbed pages on the options dialog.
In normal use the extension open a new Firefox tab and requests the online service to validate the pages. However, the extension is fully integrated with the desktop tool, so if you have this installed you can validate pages locally instead, and have access to many more validation options, such as validating a whole website with just one-click.