After a few false starts, had a great audio call with a friend over Riot/Matrix. Had some issues with video, so we might try that again another day. No luck with meet.jit.si unfortunately. Also tried Riot on mobile with me over cell data connection and friend on WiFi - audio quality not quite as good, but still very acceptable.

To get all this working, it turns out I had to make the following changes to IceCat 52.3's default settings:

media.peerconnection.enabled = true
media.peerconnection.ice.default_address_only = false

Worked on both Trisquel and GNU GuixSD. Here's a related IceCat mailing list thread.