In my opinion Section 18 contains not correct logic.

• User prompts that are spawned from beforeunload event handlers, are dismissed implicitly upon navigation or close window, regardless of the defined user prompt handler.

Example:
If we have onbeforeunload event and user try close window:
In chrome 68.0.3440.106 user should interact with promt before window will closed.
In firefox 61 user should interact with promt before window will closed.
In firefox 62beta user should interact with promt before window will closed.
In firefox 63nightly user should interact with promt before window will closed.

But in automated test with geckodriver we have different behavior.
geckodriver 0.21.0 silently set dom.disable_beforeunload to true. (in firefox 61, 62, 63)
and when test close window promt not showing and window closed.

It is different behaviour.