This is a dumb 15 lines plugin that makes Firefox open all windows in fullscreen mode. On startup it simply updates the current window to set the window state to fullscreen and adds a listener to update subsequent new windows.
Credits go to tazeat. His add-on turns fullscreen mode on only on startup, though he proposed a solution that makes it work with all windows (even after startup). This plugin is just a fork with that solution (though there aren't any "forked from" flags).
Found a bug? Please feel free to create any issues or submit a pull request.