The assumption is that two clients are in the same LAN they would use UDP anyway. If are in two differents NATs doing traversal with TCP isn't support. I have read that it is possible, not something I have tried myself though.
For 1:1 calling, I suspect the number of peers that can do peer to peer TCP, but not UDP is so small that it's not worth bothering.
But that doesn't answer your question, sorry. I know the application I work on doesn't listen for incoming tcp on clients, but I think there is support for it in the WebRTC library. (Not sure if Firefox pulls that in, or wrote their own implementation?)
But if you're on public wifi, chances are you can't get inbound tcp to work. And the majority of the peers you might connect to probably can't either. It's not worth the effort to eek a tiny % more peer to peer when you have to build the relay server fallback anyway.
I know Firefox now supports outgoing TCP ICE, but what about incoming TCP connections? I wasn't able to find an answer easily from googling.