After lots and lots of searching, there’s a simple answer:

from selenium import webdriver
from import By
from selenium.webdriver.common.keys import Keys
from import WebDriverWait
from selenium.webdriver.firefox.options import Options
options = Options()

options.set_preference("browser.fullscreen.autohide", True)
driver = webdriver.Firefox(options=options)

This will set the browser.fullscreen.autohide config key in about:config in Firefox to True.

When you call the method fullscreen_window from the WebDriver, it will hide the pesky elements at the top of the window.

I will thank myself for this post one day.