It needs you to set "full-screen-api.allow-trusted-requests-only" to false in about:config for it to work. But it does work, thank god. aaaaaaaaaa

The developer should add that it needs full-screen-api.allow-trusted-requests-only to be set to "False". Anyway, thumbs up for making an addon that finally circumvents Mozilla's failing F11 handling

I need to have the window active (i.e. click on the webpage) for it to work. But works as advertised so I am satisified.