Firefox on Android doesn't do full screen, you have to use Google's Chrome browser or a Chromium based browser.
In Chrome you can save the page as a home screen shortcut, then launching that shortcut loads up fullscreen.
Edit: aaand just noticed Kent already posted that
In Chrome you can save the page as a home screen shortcut, then launching that shortcut loads up fullscreen.
Edit: aaand just noticed Kent already posted that