I think you make some assumptions. False ones.

1. Mozilla do not develop Thunderbird. They ceased their participation in 2012. 2. Thunderbird is built on what is euphemistically called the "mozilla Platform" so while the development process of Thunderbird is independent. The developers do have to take into account when Mozilla rips great swathes of the platform out in the attempt to become relevant again. A very large part of Mozilla's decision in 2012 to withdraw from Thunderbird was their intention to over time remove almost all of Thunderbird underpinnings. So if Mozilla puts in blocks on downgrades, Thunderbird is almost required to follow as it is a core requirement. Having said that, the Thunderbird developers are not resistant to the change. It means that upgrades to profile do not have to be able to support a downgrade. V78 for instance completely reformatted the address books. Once updated they are not in any way compatible with earlier profiles. So which would be preferable. A downgrade with the loss of your address books or no downgrade. Personally I have been through the 78 to 68 downgrade. It is not for the feint hearted and is is a far from satisfactory result when you get there.

Perhaps a more constructive approach would be a more informative upgrade that allowed to user to backup when they did not like what they were seeing. But who reads such things? This forum is full of people complaining that when the product said it would remove something, and did, they were distressed because they did not actually read the dialog. They clicked ok.

I do not have the answers, but I do not think that the developers of this product will even consider a downgrade option. I have lost that argument far to many times over the years. I get such bland statements as " we owe it to our users to only offer the most secure product we can" and "old version are not supported and have known vulnerabilities that will not be fixed".

What is it that prevents you from using version 78?

Regarding Matt's point 1, see https://www.thunderbird.net/en-US/about/

Thank you for the update on the Thunderbird - Mozilla connection. Busy on other tings, I hadn't picked up on that. To be honest I am of a similar mind that things will not change. I have a software development background myself and have seen the development approach where the users are largely irrelevant and whether a piece of software is good or not is solely in the mind of the developer. It works sometimes, but it has a big risk of having the users just go away. In the commercial environment, there's usually intervention before they do. I don't know where the developers are taking Thunderbird, but in the process they've lost me. I've been using Thunderbird for years, just wanting a stable email client that wasn't subject to the Microsoft "Change for the sake of change". In Thunderbird, I'm now getting Calendars and other things I don't want. To get what I want, I think I'll have to go to a paid email client, where there is a clearer focus on providing reliable core functionality. Contributing to Thunderbird is out of the question.

check & see if these are of any use for your choices/directions: Disable AV & try TB again, TB problems & solution options: downgrade, second-TB, or transfer-data, Downgrade TB in windows.

how Mozilla killed their own limb off by discontinuing their fully working Firefox (pre-57) & unlimited addons, was just jaw dropping , never seen such before. Mozilla gets triple-digit $$$ MILLIONS of dollars from Google/Alphabet & etc, EVIL^{1, 2, 3, 4, 5, 6} entities, so obviously those $$$ has lots of effect. sameway, how it was possible that while even basic upgrade functionality remained incomplete or weak, and still was released in www via "release" channel, is amazing.

Re: "What is it that prevents you from using version 78?" As a user, my opinion on the look and and feel of 78 is worse, and it has broken compatibility with KeeBird. The Thunderbird update process isn't simple anymore. Users have to check ho many things the new version will break before upgrading. Having to do that research is a pain. The alternative of accepting the upgrade and then reverting when breakages are found isn't supported. To sum it up, this last upgrade was really unwelcome and it's likely to happen again on the next upgrade.

Re: "What is it that prevents you from using version 78?" As a user, my opinion on the look and and feel of 78 is worse, and it has broken compatibility with KeeBird.

Unfortunately for users of that add-on, the author has not been willing to invest time. OTOH, https://addons.thunderbird.net/en-US/thunderbird/addon/keebird/ indicates there are less than 300 users. Compare, Thunderbird has tens of millions of users.

The Thunderbird update process isn't simple anymore. Users have to check ho many things the new version will break before upgrading. Having to do that research is a pain.

Agreed, being able to determine in advance would be helpful.

The alternative of accepting the upgrade and then reverting when breakages are found isn't supported. To sum it up, this last upgrade was really unwelcome and it's likely to happen again on the next upgrade.

The decision to not support rolling back 78->68 comes from several reasons, but ultimately the killer is the extreme complexity of process (as you can see in places that list the full process) which is undoing the address book updates, undoing addons, undoing changes to filters, undoing the enigmail conversions to name a few. The investment in manpower (and the possible delays) to provide this simply doesn't warrant the effort for a small percentage of users that might benefit.

As for add-on compatibility, add-on authors bear ultimate responsibility for whether their add-on lives or dies. Whether they step up is not Thunderbird's responsibility, and users who choose to use add-ons for better or worse put themselves at the author's mercy. That's not a great place to be, but that's the way it works. However, one helpful change with version 78 is a Thunderbird staff member was hired several months ago to assist authors who seek help. That effort has been successful in bringing many add-ons into the current state of the art. It has also resulted in many add-ons being adopted by new authors when it was clear the original author wasn't going to provide an update for their users.

As for the bigger picture of add-on futures - Thunderbird has been playing catch up. If Thunderbird had followed Firefox precisely, then the add-on losses would have been fully incurred in version 68. But Thunderbird decided to provide a bridge in TB68, allowing many add-ons to continue to work. TB78 is the end of that bridge, so yes many now do not work. But going forward, add-ons that do implement mailextensions methods should be safe for many years to come.

So no, this situation is not likely to happen again.

As a user, my opinion on the look and and feel of 78 is worse,

You might try https://addons.thunderbird.net/en-US/thunderbird/addon/phoenity-icons/

That’s about the best justification I’ve ever heard.

But the reality is that it’s one click to upgrade and with no warning there is death by a thousand clicks, with no possibility to return to the previously working system.

From what you say, Thunderbird was aware of the potential when the hidden bomb was released with no solution to repair the damage.

The only way out for any user where the bomb exploded is to accept the thousand clicks and migrate all emails to a platform where hidden bombs don’t arrive in upgrades.

But, sure, the justification sounds like a pleasant solution for Thunderbird. There will be plenty of happy hours to discuss what responsibilities to accept or reject, as users are driven away.

Ultimately users must decide if they trust Thunderbird with their email. My answer is obvious.