An updated version of the "YouTube_FullScreen_Fix" user script can be found here:
http://morph.zone/downloads/YouTube_Fullscreen_Fix.zip

This adds support for the mobile version of YouTube.

For users who had the old script enabled previously and overwrote their copy with the updated version, you might have to manually add
Code:
to the list of affected URLs in the "Scripts" window.

If anybody keeps getting forwarded to the desktop version of YouTube, try visiting this link: m.youtube.com/?persist_app=1&app=m