What I believe Slack's XMPP server sends:

<iq xmlns="jabber:client" from="testteam-mikef.xmpp.dev.slack.com" to="newxmpp@testteam-mikef.xmpp.dev.slack.com/Thunderbird" type="result" id="b4633a7d-f172-5349-9e0f-5b85ec6518c4"><zdef1647982679:bind xmlns:zdef1647982679="urn:ietf:params:xml:ns:xmpp-bind"><jid>newxmpp@testteam-mikef.xmpp.dev.slack.com/Thunderbird</jid></zdef1647982679:bind></iq>

What's printed by the Thunderbird debug log before it discoonnects:

<iq xmlns="jabber:client" from="testteam-mikef.xmpp.dev.slack.com" to="newxmpp@testteam-mikef.xmpp.dev.slack.com/Thunderbird" type="result" id="b4633a7d-f172-5349-9e0f-5b85ec6518c4">
 <zdef1647982679:bind xmlns="urn:ietf:params:xml:ns:xmpp-bind">
  <jid xmlns="jabber:client">
   newxmpp@testteam-mikef.xmpp.dev.slack.com/Thunderbird
  </jid>
 </zdef1647982679:bind>
</iq>