I'm afraid I was disappointed at Timotax saying that Firefox "can't do nothing (sic) to help". I have managed to locate in my profile for Thunderbird Imap files labelled INBOX and INBOX-1. Presumably one of these is likely to contain the information I wish to recover. Unfortunately, opened in Wordpad they contain so much formatting information as to be incomprehensible. I would have hoped that Firefox could offer some help in interpreting them.

Perhaps some education to start. You have apparently been disappointed, but your disappointment originates with a lack of knowledge about whom does what.

You are asking about Thunderbird, it is an email client. Thunderbird is a community project run by volunteers under the auspices of the Mozilla foundation, they provide a legal home. Nothing much else, other that the use of their support web site for Thunderbird and the use of the Mozilla Thunderbird trade mark.

Firefox is a web browser manufactured by the Mozilla Corporation. A wholly owned, for profit company, of the Mozilla foundation. I really don't know much about it as an entity. My focus is Thunderbird.

While there is some common code in the projects, they are neither developed or managed by the same people. Thunderbird certainly inherits some changes made by the Mozilla Corporation in their moving of Firefox forward. But the two groups are completely separate.

Whomever it was telling you Firefox "can't do nothing (sic) to help" was quite accurate really. They (Mozilla corporation) do not build, maintain or support Thunderbird. So there is nothing the Firefox folks can do to offer you support. It would be like going to Microsoft asking for support for Thunderbird. They do similar products and understand some of the concepts. But they would not be suggesting they could help you either. Most Mozilla corporation staff do not use Thunderbird in their work, so do not even have the perspective of "fellow user" to start from. Mozilla corporation use Gsuite as their corporate desktop office suite. Not Thunderbird)

Now if you want to do something about recovering your data, make sure Thunderbird is not running, then make a backup of the two files you have located. they will have no file extension, but will usually be accompanied by an MSF file of the same name. The MSF file contains nothing useful in the recovery process.

Open the files in Notepad, or another text editor such as notepad++ if the files are too large for notepad.

Do a global search and replace on X-Mozilla-Status: 0008 with X-Mozilla-Status: 0001

Save the changes Open Thunderbird. Your mail should be back, unless there has been a compact action since you deleted your mail, but in that case you would not have changed any status information in the global find and replace. (Ctrl+H in notepad and notepad++)

Make sure not to actually save the files in a non text editor such as wordpad. It will irrecoverably mangle the files for you and I. Recovery would be possible but would require some expensive programming.