Seamonkey: Enabling full-screen for Youtube video

Seamonkey default settings

For some reasons the internet browser Seamonkey decided to prevent people to open full screen YouTube video - maybe they were tired of complaints of user who couldn't not exit the video (by simply pressing Escape?!)

Anyway now by default the full-screen button on the lower-right is disabled.

Overriding the setting

in the url path type: about:config then confirm that you know what you are doing

then look for full-screen-api.enabled and set this to true

That is it