Bottom line is Thunderbird is built on the Mozilla platform. Add-ons are built into the Mozilla platform. Therefore a change to the platform affects both Firefox and Thunderbird.

The Thunderbird developers have resisted the change implemented in Firefox by back-porting a lot of stuff, but the development burden to keep something the Mozilla developers have decided will go is more than they can maintain in the long term. So a two year plan was implemented (Ending with 78) to allow addon developers time to learn the new system and modify their add-ons. Some chose to take their bat and ball and go home, other still struggle with what is a leaner less robust system to access Thunderbird's functions. So have completed the transition.

But it is not Thunderbird taking them away, on the whole it is developers walking away that is the issue. I can not wind back the clock. The choice was go with the flow of close up the Thunderbird shop. The issue was that significant. It played a big part on the Mozilla decision to withdraw from Thunderbird development in 2012. They had no future plan for Thunderbird and had no intention of investing the time and money into Thunderbird that would be needed to keep it running on their platform with the changes they envisaged. As we near the end of those original planned changes, Thunderbird is doing better than t has in all the time since 2012, except the add-on environment has taken a beating. The project recently hired a developer to wok on the addon environment as it is still not as good as it it desired, but from here things should only get better and the old days of add-ons just stopping on updates should be behind us..

It would be better to not base it on the Mozilla platform and continue to support Add-ons. As I understand it from Firefox, it's not a matter of "allow addon developers time to learn the new system" since the new system is fundamentally incapable of doing the things that they need.

So we should just install 52.9.1 and never update it? Is there a later version that supports the useful add-ons?

re :It would be better to not base it on the Mozilla platform and continue to support Add-ons.

It is not that simple. Thunderbird had no choice. It has to go down that path in order to survive - there was no alternative. And it was not sudden. This has been known for years. But it is still not that simple for some authors of addons, who get little if any income from people who use their addons. You would be amazed by how many people use addons and do not make reasonale donations for their maintenance. The authors often simply do not have the time that would be required to learn all the new code.

https://blog.thunderbird.net/2017/05/thunderbirds-future-home/

You are using a version of Thunderbird Version 52.9.1, which first offered to channel users on July 10, 2018. So it now over two years old.

You can easily check to see what is going on with all the addons you use. It will state on the addons download page what versions they are all designed to work on. Some people are still working on their addons to get them ready for 78, so even if they do not work at the moment, it does not mean they will not work in the future.