> I didn't back up before installing.

Indeed.

> I was having significant problems with Thunderbird version 68.x.x

Please describe.

> now find I've lost ALL of my address books

How many address books? And all your email is there?

After one of the more significant updates a few months back, it screwed up the colours of the different panes so that mail could barely be read. I HAD been using a theme which was then disabled and there was no way to get the colours changed.

Saw mention from a few other users that they had some of the same related issues and had switched to the 64-bit version which had allowed them to once again make colour changes to backgrounds and fonts. So of course I followed suit, and this DID sort out those issues.

However, when I went to write an email recently, I noted that it wasn't pulling up the associated info such as names when I entered an email address when writing a new mail. Upon further inspection, I noted that all my address books were empty aside from the latest email addresses I had sent mails to. I had a 'Personal Address Book' as well as the standard 'Collected Addresses'. The address books are still there but are now empty.

Visit Help > Troubleshooting > about:profiles

What profile names do you see, and which one is listed as "Default profile"?

It shows the same profile for the Root Directory (C:\Users\Administrator\AppData\Roaming\Thunderbird\Profiles\fopzyf6w.default) as the Local Directory (C:\Users\Administrator\AppData\Local\Thunderbird\Profiles\fopzyf6w.default)....and the profile shows as being the default profile. No other profiles are present.