Sorry you are having trouble. I have updated your topic title.

Lots of problems: (1) I got a message that I was running our of storage space. Would I like to compact the emails now? Yes, I said (foolishly). (I've never compacted emails before.)

The dialog is not about running out of space. It is simply about freeing up "some" space". See https://support.mozilla.org/en-US/kb/compacting-folders/

(2) After some time, I discovered I had lost all my local folders (approx 100 folders). (3) It was then I found out that my external drive backup had given up the ghost in May (damn). (4) I then received an update for Thunderbird, and stupidly downloaded it thinking it might solve my problems. Continuing ..... (5) I found an older profile under About profiles and set it to the default. (6) Now I have these problems: The local folders are not up to date (some are missing) and therefore all emails in those folders, in the last few months, have vanished. (7) The missing emails are there in "archives". But as there are thousands of them, it is tedious to individually copy them back to their local folder, or individual inbox. (8) Additionally, my contacts in my address book disappeared. I found them in an earlier default profile, but I cannot copy them bulk into the address book. I have tried copying over the address book file from the older profile to the one I am using. Hasn't worked. (9) Have spent 3 days on these problems.

Was your Thunderbird profile on the external drive? Or parts of the profile?

I was only able to access a profile in my external drive (backup), which I copied back to my hard drive. I did not find parts of the profile. I was able to search for the address book, and the local folders, but again that was in the external drive and was dated. I think I lost the current profile, address book and local folders when I update the Thunderbird software (as stated in my original question). I have not restored the emails from the archives to their inbox folder, because there are 6800+ of them. As to the local folders, I am trying to rebuild them. As to the address book, I am trying to rebuild it. In both cases above, I am referring and searching archives.

If you were in a perceived pickle before your fix has at best made it worse. How? because you are not trying to integrate parts of a profile. The upgrade process from 68 to 78 changed the name and the format of the address book files. So depending on the age of your old profile a copy is not going to fly.

I will not address your attempted fix because to be honest I have no idea what you have done. So I will not look as 6 - 9 as they are not really the issue, only issues you have from changing to an older profile.

Lots of problems: (1) I got a message that I was running our of storage space. Would I like to compact the emails now? Yes, I said (foolishly). (I've never compacted emails before.)

I think Wayne has covered this issue

(2) After some time, I discovered I had lost all my local folders (approx 100 folders).

Lost how? What do you mean here. Are you saying that the parent at the top of a hierarchical group of folders is missing or a list of 100 folders is not displaying? While similar problems it is difficult to be exact with only imprecise information is available. However,, deleting the file foldertree.json from the profile folder without Thunderbird running can often help with incorrect folder displays.

If you use he unified folder view a different (somewhat) set of files have to be deleted as well in the folder containing the fordertree.json file open the mail folder and delete the folder "smart mailboxes"

(3) It was then I found out that my external drive backup had given up the ghost in May (damn).

Fairly typical of automated backup is my experience. With out regular attention that just stop working, usually exhausting the available space if nothing else.

(4) I then received an update for Thunderbird, and stupidly downloaded it thinking it might solve my problems. Continuing .....

While it would be unlikely to fix an ongoing issues of this nature it is also most unlikely to make it worse.

(5) I found an older profile under About profiles and set it to the default.

So perhaps the solution is to just go back to the other profile so we can work on making it functional rather than messing with something that is out of date.