Rated 5 out of 5 stars

As the title says, I can't go fullscreen on embedded youtube videos. It just says fullscreen disabled.?

This appears to not be SeaMonkey's fault - at this moment embedded videos don't work in other browsers as well, not even in Chrome. Maybe some temporary technical problems on YouTube?

If you find a video that works full screen in other browsers but not in SeaMonkey then please report it to my contact email.

Rated 5 out of 5 stars

As for those saying it doesn't work,
type "about:config" in the location bar,
at the message warning just hit "Enter",
then in the search bar type "full-screen-api.enabled" and change the value to true.
Now youtube fullscreen will work.

This is not necessary because this extension already sets full-screen-api.enabled on installation and on every browser startup. If someone has problems they must be caused by something else.