It really is a question of what is a conversion especially as some web mails do not include the necessary references to links them up.

Have a look at message source at the References:, In-Reply-to: and message-id: there are the fields that are used to pull all the bits together to make a conversation. are they cross referenced in the various mails you think are in the conversation?

I'm talking about the "Open in Conversation" option under the "More" tab within the email. Please see image attached. It used to work fine and I used it regularly but now it's greyed out and cannot be selected.

So am I. the method of determining a conversation does not change regardless of where you look to use it. It is also n the right click menu and the message menu or it can be invoked with the shortcut Ctrl+Shift + O but if the information is not in the header then it will be disabled.

Oh dear, I think we speak a different language. I just need someone to talk me through, step by step, without having to get into source code, how to fix this issue. Thanks anyway for taking the time to try and help me.

I doubt there is anything to fix. Sure it is greyed out. That is the case for almost all email that do not contain that references header. All I am asking to to do is look and see if it exists.

I see, thanks. So what do I need to do? I'm sorry but I don't understand "Have a look at message source at the References:, In-Reply-to: and message-id: there are the fields that are used to pull all the bits together to make a conversation. are they cross referenced in the various mails you think are in the conversation?"

many thanks

When you have an email open you should see at the top of the window a series of buttons like 'reply' 'forward' 'delete' etc. At the right there should be another button that says 'more' which should give you the option to 'view source'. Click on that and you will see displayed (in a new window) the text of the message preceded by a whole lot of information. That information is used by computers to route and keep track of emails as they travel around the world; it's normally kept hidden from you because you don't usually need to see it. What Matt is telling you is that some of that information is used by Thunderbird to infer that the message you are looking at relates to other emails that you have previously sent and received in what we humans call a thread or conversation. (Computers themselves don't have that level of understanding, obviously.) And he's saying that if that information isn't complete, Thunderbird's attempt to relate this email to others will fail, and it won't regard this email as part of a conversation -- so it won't offer you the option to view it as such. As to why the information might not be there, I have no idea: there are several possible causes. The system depends on senders having their email accounts configured to send full information in the first place; on using the 'reply' button to reply to a message rather than composing a new message that is (in the sender's mind) a reply; and so on. I hope this helps you understand the issue. It obviously isn't a solution to your problem, and I doubt that there is one.

Matt said

It really is a question of what is a conversion especially as some web mails do not include the necessary references to links them up. Have a look at message source at the References:, In-Reply-to: and message-id: there are the fields that are used to pull all the bits together to make a conversation. are they cross referenced in the various mails you think are in the conversation?

@Matt, I checked and my newer email has the MessageID of the older email as a References.

See http://kb.mozillazine.org/Stop_threading_by_subject

If there is nothing there that helps, restart with add-ons disabled from the help menu and see if that helps get it working.

Thanks for your explanation Aman. I have clicked on View Source, now what do I look for?? I think I understand what you and Matt have both been saying about the information needing to be relayed in the msg header, but I'm not sure why it just stopped working. It used to work with every single email conversation I had and was super helpful.

Attached is a sceeenshot of the top of a recent email's source code. Are you able to tell what's missing from it and how I might be able to fix it?

I've tried restarting with add-ons disabled before (when finding that as a possible solution with a google search) but it didn't fix the situation.

Thank you!

I am leaving this thread since I've opened another one https://support.mozilla.org/en-US/questions/1115166