Thunderbird 78 IS NOT getting pushed to Tbird 68 installations. You chose to jump the gun and installed 78 nonetheless, without a look at the release notes. Not even a backup of your profile is in place!?

Well, that comes to another weird numbering of versions. I'm using Ucheck which told me to upgrade. But, I had no idea that if will change my database. Yes, I have backup, but I'm not making backup everyday.

Again, I went to the official Thunderbird downloads. https://www.thunderbird.net/en-US/ All version 78 ! No link for older version, no warning.

Thunderbird has its own automatic updater, so you can't fault it when you choose to use a third-party utility like Ucheck. Version 78 is the new release, and it is NOT an update to version 68. They are two separate product releases. Old versions are available here https://archive.mozilla.org/pub/thunderbird/releases/

I'm using automatic updated, but I was tired and made a mistake. BUT, when version 78 installs it can check if it's fresh install and if not, put the warning that the database will be changed. It would be big improvement and would stop others making the same mistake I made. Is that really that much to ask ?

That wouldn't help during installation, because starting with v68, each product release uses a dedicated profile. When I installed v78, it did not use the v68 profile when I launched it. It created its own blank/fresh profile as expected. Loading the v68 profile required additional and conscious actions from me. A warning at that point (loading an old profile in a new product release) would be more helpful though.

Just not true. It changed my data after installing without me taking any action after install. Warning came, but way too late. Warning is you data is ruined continue or make new profile. It changed my profile after I installed v78. Didn't ask which profile or anything like that, it just fucked it up. Sorry, but that's just doesn't make sense. Why would I need new profile ? As I said, if you install v78 fresh without having TB installed, it makes sense. Otherwise it doesn't. Programmers are responsible for user data as well.

When you have third-party programs taking care of installation of Thunderbird, it's hard to tell what other "smart" decisions those programs make. I had 68 on my system when installing 78, so it wasn't a fresh install. When I launched 78, it started with a blank profile of its own, and it didn't touch the old profile that was used with 68. This is by design, unlike in Thunderbird versions before 68, which used the same profile. This new design allows you to have multiple versions of Thunderbird installed on the same computer, without affecting the profiles used by each version. I have both 68 and 78 installed, and each one has its own profile. 78 did not touch my v68 profile, likewise, v68 does not at any point attempt to load the v78 profile. This is covered in detail here Dedicated profile per Thunderbird installation, which I contributed in. It effectively prevents accidentally upgrading a profile that is in use by another installation. If this new behaviour does not showcase TB's programmers caring about user data, I don't know what else does. Because of this behaviour, we are getting reports from users who think their data is lost when they installed 78, because 78 loads a blank profile when launched and ignores their old profiles, exactly as programmed to do.

What you can do to get your ImportExportTools NG add-on back, is to install v68 and restore the most recent backup of the v68 profile, or keep using 78 until the add-on is updated to work with it, which is (a lot of) work in progress by its developer. It will also do you good to exclude Thunderbird from Ucheck's activities, because it basically bypasses the official schedule of how updates and upgrades are offered/delivered to Tbird users.

There are no other "smart" decisions UCheck program makes. It reports updates. 99% of the time it just reports me which updates exists, and I check them on the original website etc. This case was just slip on my part, because being tired. Also it didn't report v78 until recently, so there is certainly something changed. What exactly happened and in what order is pointless now since I can't prove it at the moment because it's done already. My point which you missed is that it should warn you BEFORE installing 78, not after. That's ALL. How difficult is that I don't know, but I think if checking why not check before installation ? This is my advice so that people would avoid doing the same mistake as me. You don't have to agree, but I think it's only reasonable action if you really care as you said. Also I noted, for a looooooooooong time I've used thunderbird only and never had big issues until recently.

I followed the link didn't find what are the changes in the database and how to get it back. I will have to wait for the extension update, my database is too big to play and experiment with it.

I will have to wait for the extension update

Yes, the author of the add-on is an independent developer voluntarily working on it in their free time. They are aware and have assured us that they are dedicated to working on it. Note that they are solely responsible for making sure their add-on is compatible with Thunderbird. It is not a core component of Thunderbird, so it doesn't follow Thunderbird's development roadmap. Perhaps in future the developer may collaborate with Tbird's developers to make it an integral part of Tbird, but until then, it remains a separate development and your decision to use it should be informed of that fact. Changes to data file formats are inevitable in software development and are decisions that are arrived at after a lot of consideration. You agree to those changes when you choose to install software upgrades that implement them.

We could debate all day about whether a warning during installation would help, but in all honesty, would you have paid much attention to that warning in your "tired" state? Mozilla is being careful not to automatically upgrade v68 users to v78 and this has been implemented on the update/release servers. On the website that Ucheck lead you to, there is an announcement blog post for v78's new features. Not much attention is given to it by users, instead they would rather hit the download button without much thought, then complain later when they don't like the changes! Developers can only do so much. In that blog post, it is clearly stated:

As with previous major releases, it may take time for authors of legacy extensions to update their add-ons to support the new release. So if you are using add-ons we recommend you not update manually to 78.0, and instead wait for Thunderbird to automatically update to 78. We encourage users to reach out to their add-on’s author to let them know that you are interested in using it in 78.

As expected, complaints are coming in after the fact, from users who didn't give much thought to it before upgrading their Tbird installations. What would really help users, is for them to actually care about these announcements before installing new versions of software they use. It's very little investment on the users part, but nooooo, they wouldn't want to deny themselves an opportunity to rant about their shortfalls. They have their own solutions in mind, but when things don't go as expected, it's always someone elses fault!

I have ver 78 and just want to sync my mail rules on two computers.