More news from TenFourFoxBox land: I have managed to get it to work with Cappuccino. The first time I tried I used nice.social as the website. It didn't work, just showed up as a generic TenFourFox window. Subsequent attempts with chat.10centuries.org achieved nothing because to get rid of a faile Box, you have to get into your user Library & delete references to the "Box" in Applications Support & Caches.
Once that was done, tje proper chat.10C was enabled.