If you need to validate flash or other embedded stuff !
Firefox does need the embed tag, to display flash.
But if you use this tag, your page will not be valide XHTML!
My way to solve this is the so called AJAX technology.
I take all the embbed-code and include it with an asynchronous request
into the page. The w3c-validator parse the site without doing this.
Nothing is really valid, but the validator disagree!
The w3c-validator does not find any problems in this page anymore.
If this has not been wholly helpfully, write an email.