I keep seeing this question around and am wondering if anyone has had any luck? I tried an add-on, but that didn't work. All the other stuff (script, etc.) is beyond my comprehension. Will Firefox ever add this feature? Or, would someone be willing to do a step-by-step?

this is what i'm talking about

it seems full screen on my mac catalina o.s.

what i dont see on your end is the menu toolbar and then the options on it that offers full screen.

You can check some prefs on the about:config page and try to toggle them to see if that has effect.

Whether to use macOS native full screen for Fullscreen API

• full-screen-api.macos-native-full-screen

Whether the toolbar should slide down with the menubar when the user mouses to the top of the screen in fullscreen mode.

• full-screen-api.macos.shiftToolbar

You can open the about:config page via the location/address bar. You can click the button to "Accept the Risk and Continue".

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

If you right click/2 finger click near your address bar where there are no buttons, a menu drops down. On the drop down menu, you can opt to hide toolbars. This is native to firefox with no extensions installed.