How can I completely prevent firefox from showing the address bar in fullscreen mode

On new Firefox v.71 currently on Nightly (Beta) you can add --kiosk when running to avoid all bars (navigation and tabs)

https://www.mozilla.org/en-US/firefox/channel/desktop/#nightly https://www.ghacks.net/2019/10/07/firefox-71-new-kiosk-mode-for-the-browser/