I tried it briefly and disabled it. It cannot send the code of a page from my local/development server to the W3C validator. It's only useful for pages that are already live-online (public), but that's not when I do my code validation; I check my code while it's still in development.

You might think it's impossible to send non-public code to the W3C validator, but I do it all the time (by Command-U to see the browser's parsed/rendered code, copy all, go to W3C, and paste into their validator). If I can go through these steps, why can't this extension do that for me?

Another (minor) problem is that there doesn't seem any way to have this extension put an icon in the Firefox toolbar, like my other extensions do. (Yes I know how to customize the toolbars.) This extension seems to be only accessible by pulling down the Extensions sub-menu.

Not worth the trouble. Disabled.