On desktop, does it still require installation of this addon to get the UI bits? https://addons.mozilla.org/en-US/firefox/addon/send-tab-to-d...
EDIT asking because it's been this way for a long time, see https://wiki.mozilla.org/Services/PushTabToDevice#Desktop_Im... which says "Currently, push-tab-to-device on desktop must be used through an addon: [send-tab-to-device]"