Why not Selenium?

Marionette is plain TCP with length-prefixed JSON, so the client here is about a hundred lines of standard library. No geckodriver to keep in step with your Firefox version — a common source of CI breakage — and no dependency to install for the core commands.

More importantly, screenshots are taken in Marionette's chrome context, which captures the browser window's own document. An ordinary WebDriver screenshot only captures page content, so toolbars and tabs would never appear at all.