How do I manually THREAD several messages that I know are related?

Threading in Thunderbird and most other email clients depend on the messages' "Message-ID" and "References" headers to know to automatically associate them into threads. Threads can easily be broken by the sender, for example if they try to contribute to the thread but don't actually reply to a message within the thread. In that case, Thunderbird doesn't have any way to know they're part of the same conversation. In your case, where you know there are disparate messages that are actually part of the same conversation, there's no real way to associate them. But I can think of a couple ways to get a similar effect... - copy the relevant messages to a folder. That would mean you have duplicate messages, maybe not optimal. - use the Tags feature to group messages into categories. Then use the Quick Filter to isolate them for viewing. - If all the messages are in one folder, and you can build a query using the "Find Messages" feature (see <Edit | Find | Search Messages> or <Ctrl+Shift+F>) that finds them all, you could save this as a Virtual Search Folder>. - I found this add-on, "Thunderbird Conversations" (https://addons.thunderbird.net/en-US/thunderbird/addon/gmail-conversation-view/). I don't actually quite understand what it does, but it's something you can investigate.