It is the attempt to group like emails together. It can be done by subject or with a reply sequence.

A thread is a link between emails like a continuing conversation. Someone sends you an email and you click on Reply to repond. The email you send back is using the same Subject as reference. This 'Replying' may go on for a while, until the topic is ended. So, you may want to see all of these received emails grouped - threaded.

In the email list, you can click on the 'threaded' column header icon to enable or via View > Sort by and View > Threads.

You will note that threaded emails in the list are shown with the first email at the top and subsequent emails below, so that the most recent is at the bottom. Sometimes this can make it difficult to locate recent emails.

You can also right click on email and choose 'view in conversation'. this will open in a new tab and show not just the received threaded emails but also show all your sent reponses. This is the best way of seeing all emails in the conversation. In this tab you could then select to remove the sort by threaded and choose to sort by date, descending. This will then list the entire conversation with newest on top.

You can choose to ignore threads or watch threads: https://support.mozilla.org/en-US/kb/ignore-threads http://forums.mozillazine.org/viewtopic.php?f=39&t=2784359

Thanks to Toad-Hall for the reply, now I just need to practice!