It will help to know the explicit steps you took, as what you describe seems highly unusual. Is the old computer still available and does Thunderbird run on that computer? I ask that in case we need to start over. Thank you.

Thanks for your reponse David. What I did was go to the AppData folder and find the Thunderbird folder. I upkoaded to cloud, downloaded to new computer to the same directory location. I then ran Thundebird. I found there were thousands of emails all in the inbox, but it had not copied over all the folders and subfolders. Checking the emails I saw that the oldest was from 2023, whereas my orginal Thurderbird has emails going back to 2002.

Yes it still works on the old computer. The old computer is running Windows 10 and the new one is running Windows 11.

What I can't remeber is whether I used AppData/local/ Thunderbird or AppData/Roaming/Thunderbird, and I wonder whether that could be the source of the problem?

If this is a POP account, you risk losing messages if you take profile from old computer and copy directly. Moving from one pc to another is best done by 1) exiting thunderbird and, 2) copying c:\users\<yourid>\appdata\roaming\thunderbird to external media, 3) installing thunderbird on new pc and exiting, and 4) copying the backed-up thunderbird folder to c:\users\<yourid>\appdata\roaming. The missing folders can be copied over using Windows File Explorer. Exit thunderbird, locate the missing folders on old PC, most likely in the Mail\Local Folders folder, copy them and paste them into the same folder on new pc and restart thunderbird.

Thanks again David. What you described is exactly what I did. As I have explained it only worked partially and only included emails that were less than two years old.

By "missing foders" I was referring to the folders one creates within Thunderbird's "Local folder" and where emails end up if one has created filters using the "Create filter from message" option. This cannot be handled using File Explorer.

But this does suggest another approach to the missing emails. Is there any utility (whether by Thunderbird team or 3rd party) that can extract emails from Thunderbird data files and turn them into text that can be read by a word processor?

First, the folders need to be imported into Local Folders as I discussed above. Then, the importexport addon can extract all the messages into eml files. THEN, you can do web search for 'convert eml to doc' and there are several websites that offer that free service.

Okay so the way I "solved" the problem was to give up the idea of preserving all my old emails on my new installation of Thunderbird on my new laptop. I will have to live with Thunderbird imposed limit of not going back more than 2 years. However I will not lose all my old emails. In any case it's only certain important folders I wanted to keep. So on my old laptop I selected all the emails in one of those folders. Did a right click and a Save As and ended up with hundreds of EML files. I then used an online utility from a company called PDFen which converted all the EML files and merged them into one PDF file.