Firefox configuration
Plug-ins
about:config
Follow the system theme
Set this preference to make web content follow the system's light or dark theme:
layout.css.prefers-color-scheme.content-override (Number): 2
Disable fullscreen widgets
full-screen-api.ignore-widgets (Boolean): true
Disable the fullscreen animation
ui.prefersReducedMotion (Number): 1
The following two should not be required in a new install of Firefox, but in case they are set from an old installation, make sure to set them to the following values:
full-screen-api.transition-duration.enter (String): 0 0
full-screen-api.transition-duration.leave (String): 0 0
The ui.prefersReducedMotion should indeed be a Number. A Boolean value true will not work.