Try this: click IRC and enter matrix.org as the server and proceed from there.

Matrix chat support was officially added to Thunderbird in version 102. https://www.thunderbird.net/thunderbird/102.0/releasenotes/

Thanks to both of you for your replies. I've updated TBird and was able to open a Matrix chat account. However, it looks as if Thunderbird isn't "cross-signing capable." Is this right? My chat rooms are all marked "Unverified" and I'm not seeing any messages.

What am I doing wrong?

TIA Les

Thunderbird supports cross-signing. You'll have to verify your session though.

Thanks, Christ. I've tried to verify my session, but I can't figure out how. Can you point me to a help page?

TIA Les

Frankly, I'm struggling with Matrix chat too. Matrix feels like a weird protocol, and I don't really understand how it works. I'm using Matrix chat only for some Mozilla chat groups on chat.mozilla.org. In order to verify a session there I do need another Matrix client, which is Element in a web browser. With both clients logged in to the server, one can initiate a session verification from either end, and confirm it on the other client. It did work for some sessions but not for others. As said before, Matrix is weird. Not sure if that helps in any way, but that's all I can tell.