contextOptions.screen is ignored on Firefox. Chromium and WebKit
both pass screen dimensions independently of viewport; Firefox
drops the value and uses inRDMPane which ties screen to viewport.

it.fail(browserName === 'firefox' && !isBidi,
        'Screen size is reset to viewport');

Mozilla landed SetScreenAreaOverride on BrowsingContext
(https://phabricator.services.mozilla.com/D273425,
mozilla-firefox/firefox@a689946)
which decouples screen dimensions from RDM.