At first I thought this was a T-bird issue. But on reading some of the messages it looks like it's not.
I have a number of messages, all dated today at 11:32 AM, which should be part of threads but aren't. However, they are not the original message, rather they are some sort of echos. They start with "To: " and a name and end with something like,
--- SoupGate-Win32 v1.05 * Origin: (1:249/999)
--- Synchronet 3.15b-Win32 NewsLink 1.92 SpaceSST BBS Usenet Fidonet Gateway
Did a news server go bonkers?