When I visited the site to test there was a temporary error when I tried to load a few videos, but Flash Plugin did load and I could hit fullscreen. I know by default when I downloaded Firefox on Windows 10 that Flash was not installed. Is flash installed in your Firefox Profile? You can check the fullscreen permissions in about:config or try these troubleshooting steps first: [https://support.mozilla.org/en-US/kb/flash-videos-wont-play-full-screen]