You have used the word "archive" several times but I don't think anyone here can guess exactly what you are referring to.

I'd look at moving the profile to the new computer.

Bear in mind that if you were using IMAP, all this concern about archives probably isn't relevant, since once you connect the new Thunderbird to the accounts again their contents will just appear. However transferring your profile will preserve your address books, addons, customisations and other settings, including the account settings themselves.

If you have been using POP or the Local Folders account then you need to save your profile, because it is in all likelihood your sole copy of your old email.

https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data

I'd assume that if you had to add wifi capability to the old computer by using a dongle then it it is rather old, or is a rather low specification. I'd guess that support for the dongle is what has gone missing in the latest Ubuntu. You might be able to get it back into use by buying a newer dongle, but TBH, that is a bit of a gamble as there is no guarantee that it can be made to work, and, anyway, isn't actually relevant to getting Thunderbird to work on the new machine.

First, about 'archive'. Thunderbird email allow archiving of messages by year or month/year, making as it were a folder for each year, with sub-folders for months within a year if that option is selected - but, surely, you are aware of this. I have used MS Outlook a lot in connection with my job and in Outlook you can store email messages in separate folders (say, project folders or folders for certain correspondents). In Outlook, this basically stores a ".pst" file which can be moved around depending on where you want to store it and can be opened by double clicking it. When this is done, it starts up Outlook and opens the subject pst file with, say, all the stored inbox messages for the particular project/correspondent. I thought if i could copy Thunderbird archive folders in a similar way, this would be a convenient way of storing the messages put into the old laptop Thunderbird archives. It appears that the archive folders created in Thunderbird cannot be manipulated in this way.

However, what I gather from your response is that I can "grab" my whole profile from the old laptop and somehow copy that into my newly (some months ago now) set up Thunderbird email on the new computer. Also, I understand that this profile will contain all the messages (inbox and sent items) stored on the computer. I'm not sure how this 'old' profile would integrate with the 'new' profile now in place on the new computer.

I'm using IMAP on both computers. I really just want to scrap the old laptop, it's past it now, 10 to 12 years old, Dell, high end in its day. Not really any point in trying to get it back onto the Internet. I'm only keeping it going to keep these old emails and some other stored files I can easily transfer at any time; email messages and attachments being the only items that are more tricky to handle.

Now, I can save individual email messages in T'bird as separate files and move them around wherever I want, and subsequently open them up in Thunderbird, including on the new computer. This would be a very laborious task but if it has to be, it can be done. I had hoped it would be possible to move the archive folders around in the same way.

Hope I'm not being too much of a pain, and thanks again for your earlier response. R

The Archive folders are just folders within accounts. They don't have any independent existence so cannot be so easily picked up and moved about as you did with pst files. The profile itself is the nearest equivalent to the pst file, but you can't just add one to an existing set up. You can set Thundebird up with multiple profiles, but users do this to ensure separation between, say, work and personal email and isn't well suited to what you wish to do.

I suggest you look at the ImportExportTools add-on which lets you export some or all of your message store. IIRC, you can export/import selected folders as mbox files which is probably the most appropriate option for the granularity that you need.

If you prefer to work at file level, you can search for and copy the Archive files themselves in your file manager and paste them into the new machine's profile. You'll need to study the file structure to make sense of the relationship between files and the folders they represent. Some experimentation may be necessary to locate the best place to paste them into in order to become visible in the new Thunderbird.

You're correct in that merging an old profile into a new one is non-trivial.

I did locate some mbox files but I'm not confident enough to start messing with them. I'll have to work at file level and just copy the emails that are most important. I had experimented with this earlier and it worked OK, just very laborious. Good job for winter months! There are email attachments that are important too, I'll just go through and save what I need, that's easy! All makes me think that I should be more circumspect and methodical with email management going forward. Like Thunderbird and Ubuntu, happy to have an alternative to Microsoft.

Thanks and best regards, R