Maybe this is Mozilla's retort to Google's Chromecast, maybe it isn't. Either way, the end result certainly looks similar. Details are scarce, but an enigmatic Google+ photo shared by insider Mark Finkle clearly reveals some sort of wireless tab mirroring between a Nexus 4 and a Roku box. We're going to hazard a guess that it's being orchestrated through the web, rather than merely being based on WiFi Direct or a similar device-to-device protocol. In the same manner as Chromecast or Apple's AirPlay, this could allow the Firefox-running smartphone to be used independently from what's shown in the display -- so, for example, it could work as a keyboard or a remote control at the same time as feeding content. Anyway, there's a limit to how much we can glean from a single pic (could that be a DVD-VHS combo player on the shelf?), so we've asked Mozilla for a bit more detail and will update this post if we hear back.
Update: Mozilla has confirmed that it is indeed at working on a second-screen solution for Firefox on a range of devices:
"We are conducting some experiments around second-screen support with a number of devices. But this is at investigation stage and we have nothing to announce at this time."