I recently upgraded to a Beta version 88.

Why? Beta versions are intended for testing, not for production systems. If you deliberately install a beta, you should know how to deal with it. For everyone else, stick with the release version (currently 78.9.0). https://www.thunderbird.net/

So, I thought everything would go back to normal if I simply go back to the previous version 69.

Bad idea. Note, v69 is an old beta as well. Due to changes in the profile downgrade isn't easily possible.

This downgrade resulted in my losing everything, my complete profile, all my emails, all of my address book, everything!

Most likely upon downgrading a new profile has been created. So it looks like everything was lost, even though your data possibly still exists in the original profile.

So, I tried going back to the 88 Beta, but that simply imported my new and practically empty profile.

Thunderbird doesn't import anything. You start Thunderbird with a certain profile, and then you do see the data existing in that profile.

Why doesn't the profile from an earlier version get automatically backed up when switching versions?

That would have been your responsibility. Creating full backups of your profile on a regular basis is a good idea anyway. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

Why does downgrading destroy my profile completely?

See above.

Is there any way to get all of my data back (address book and emails)?

After the mess you created, I really don't know. First of all I'd suggest you think about which version of TB you want to use, instead of wildly switching between betas.

I am not wildely switching between Betas. I only accepted to try the 88 Beta. When that didn't work, because it made it impossible for me to send out any emails, I simply switched back to a standard version that I had before.

You said: Thunderbird doesn't import anything. You start Thunderbird with a certain profile, and then you do see the data existing in that profile. Well, if that is true, then why doesn't Thunderbird give me the opportunity to select which profile I want to use? Instead, it picks the worst profile it can find, the empty one (the one with no data in it).

Why is all my data inside a "profile"? A profile should be just my name, address, and any other relevant information that identifies me. That is totally different from my DATA (my contacts, my emails). DATA should be separate from the program and from my "profile". Data should always be accessible regardless of which version of Thunderbird I use. But, no, Thunderbird discards the data without even asking me.

Yes, when I went back to my earlier version of Thunderbird, I was warned hat a new "profile" would be created. But there was no warning that a new profile means LOSING ALL MY DATA.

I am deeply disappointed in Thunderbird. It is totally unreliable if it cannot keep all my data intact between two versions of its software.

It took me years to build up my contacts, and to set up several groups of contacts. And now it's all gone. And all because Thunderbird LOST it all because it is unable to keep it intact between two versions of its software. This is the worst software I've ever seen.

Firefox updated itself to the newest version - 89 - and with it everything came crashing down. Access to the application that I use everyday to teach and earn a living is all garbled and unusable. Gmail is all screwed up. I tried to reinstall the previous version and I lost all my bookmarks, addins, password manager .... everything is gone because Firefox decided to upgrade itself ..... Awesome!!!!! Excellent job! Keep it up!!! I am sure you can do better ....