I get that, too.

Google's explanation of the error message is: "If you're receiving the message "YouTube on TV is not supported on this device" this means that the device doesn't meet the technical requirements for running the YouTube on TV app."

They don't list desktop or laptop PCs among the compatible devices: http://www.youtube.com/yt/devices/

Not sure how to investigate further. What would be the advantage of accessing this on a desktop/laptop PC?

Its used as a plugin for Windows Media Center.

Not a big issue just means users will have to set it to use an alternate browser that does support it.

This looks like a problem with some Firefox releases. The site works with the current 34 Beta and the current Nightly 36 build, but not in the Firefox 33 release and in the current Aurora 35 build. I don't know what goes wrong as there are no error messages in the browser console and the HTTP request/response headers also do not seem to be given a clue.

Current nightly build 36 is working so definitely a bug.