The page chrome://browser/content/browser.xhtml shows the whole Firefox UI, which you can use to work on your userChrome.css, and inspect with Dev Tools (rather than the Browse Toolbox in a separate w

This has nothing to do with Google Chrome. Chrome in this case is the set of user interface elements of the application window that are outside the window's content area.

As far as I know, Firefox has been using this Chrome protocol way before Google Chrome was even a thing.