Versions 60.x, 68.x and 78.x involved architectural and technical debt changes that required changes (sometimes subtle, sometimes fundamental) to add-ons, such that they would not function unless the author updated the add-on (assuming the author was even still available and reachable). This is why there has been much turmoil for the past two years, a significant part of which was driven by Firefox changes in pursuit of greater stability and performance.

That era is done. One could go into the technical reasons for why this happened, but there is little point - it is behind us.

Going forward from version 78, there should be no point to pursuing the herculean effort you propose because ... 1) Add-on interfaces do not change during the lifetime of a major version such as version 78.x - this an add-on that already works in 78 should continue to work while using 78 for the next year. 2) For future major versions, 91.x and beyond, there are no planned changes such as those we have experienced in the prior two years.

i also do not like, the way TB devs are handling TB. releasing UNTESTED item or insufficiently tested item, in stable/release channel to people who are general users , w/o profile backup option, w/o warning, w/o ... , w/o ...

please see more info on how to disable auto-update or update in TB, here , (please upvote my Q+A inside that linked page, if its useful/helpful).

you can create different "policies.json" for different TB with different rules/policies for update or auto-update. then different TB will work differently as you've specified in their policy config file.

you can use second-TB from PortableApps project : Portable-TB , or copy all files in different folder & manually configure various settings to run it as second-TB . obviously portable-TB is very easier. please see option-2 here

TB = Thunderbird.

EDIT : updated this post , added more info.

Could this be a situation where installing the Beta build of Thunderbird side by side the release version would be helpful? It could tip you you off immediately, or at least somewhat quickly, that something has gone wrong & will go wrong in the future when that Beta build presents a new bug/problem.

https://www.thunderbird.net/en-US/thunderbird/beta/all/

I'm just not sure if it auto creates a separate profile for the Beta build. But there is a way to run multiple Firefoxes side by side, each using different profiles. I think Thunderbird has that power as well. If it does, I would just copy your current release profile to the Beta profile and use them in tandem. But to stay on top of things/breakages, you'd probably need to use the Beta build much more. Then you could report any bugs you find early. Or at least help you avoid upgrading TB until another time when the Beta shows to have resolved any issues you found.

releasing UNTESTED item

Well, this is just false. We have 50,000 beta users. And we test every build that gets shipped.

Could this be a situation where installing the Beta build of Thunderbird side by side the release version would be helpful? It could tip you you off immediately, or at least somewhat quickly, that something has gone wrong & will go wrong in the future when that Beta build presents a new bug/problem. https://www.thunderbird.net/en-US/thunderbird/beta/all/

If you want to be a beta user and file useful bug reports that would be great. And yes you can run different releases side by side, each on a different profiles. Many developers and testers do that. But you'd need to run the beta frequently enough to make it worth while. So many people only run the beta.

But this all begs the question what are you trying to solve. If it is avoiding add-ons incompatibility then you don't want to be running beta - because many add-on authors don't support beta. Plus, I have already told you add-ons that are already working on version 78 will continue to work on version 78, and we don't anticipate add-ons breaking in version 91.

So will you tell us how that won't address your concerns, or tell us which add-ons are causing you problems?

Thanks Wayne! I'm not the OP but I was just chiming in with an idea for a middle ground the user might be happy with to avoid surprise regressions. But I did forget to consider the addons this user considers critical would be incompatible with the latest beta. D'oh!

The addons the OP considers critical are Google Calendar Provider (says "Works with Thunderbird 78.2.0 - *") and GContact Sync.

I did see a workaround addon called CardBook in the reviews for GContact Sync, which is confirmed to be working on TB 78+. Review from GContact Sync: "This is a must. Every site speaks about GContact Sync for Google contacts on TB. But on 78 stopped working. A solution is switch to CardBook (personally tested). I hope to have GContact Sync working again on TB."

Hope that helps a bit. I'm not a TB guru though like Wayne & the others tho so I'll defer to you guys on this. :)

There is also TBsync - though I'm not a user of any of these, so don't know if it helps the reporter.

Since you asked for more....

It turns out that i was on 68.12.1. I found this out by looking through the GContactSync log file which recorded the version of TB it was running on. NO THANKS TO THUNDERBIRD!!!

So TB updated itself from 68 to 78 without warning me about potential incompatibilities! Not cool.

THIS IS ABSOLUTELY UNACCEPTABLE BEHAVIOUR TO FOIST ON GENERAL USERS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

1. EPICFAIL !!!!!!!!!!!!!!!!!!!!!!!!!!

I have now spent at least 4 hours trying to get back to 68.12.1 by recovering my address book from .bak file dated immediately before I recall the update being applied and breaking everything.

And NO, I will not run a separate install of TB Beta because it requires a different profile folder which means that I would somehow have to keep my TB profiles in sync. If you made this dual install and sync an automatic option, then I might consider it. But I am not interested in creating a lot of additional complexity to manage just because TB developers cannot be bothered to have the courtesy of considering the impact on general users when they design their deployment/update strategies for non-backwards-compatible changes.

While I am a very technical person, e-mail is a utility program and not something that I want to understand the technical underpinnings of. I have no interest in learning this and any effort I have to spend on keeping it working is PURE OVERHEAD AND HUGELY RESENTED! (In case you haven't figured it out yet, I am REALLY PISSED about this whole thing. PISSED at TB and PISSED at myself from not having learned from past episodes of incompatible updates that I really should just DISABLE ALL UPDATES to TB. FOREVER. Because I cannot trust TB or it's developers to keep it working through updates.

I am now restarting windows to finish deinstalling TB so I can start reinstalling 68.12.1 (again), disabling automatic updates (this has F'ED ME UP several times in this process) and recovering my address books.

I have managed to install 68.12.1. And convinced it to use my old profile via the --allow-downgrade command line option. It was busy trying to update itself to 78.5.1 when I shut off updates. (I can see that staying on 68 might be a continuous battle.)

I backed up my profile first, then deleted all the abookX.sqlite files with timestamps since the 78.5.1 update and renamed all the old abookX.mab.bak files to be .sqlite files. My mail folders are all there but the address books haven't reappeared. Just the empty personal and collected addresses ones.

Helpfully, the About Profiles page came up automatically which let me set the old profile to be the default. And TB is now successfully restarting without the --allow-downgrade option.

I had to remove and reinstall my critical extensions that had been updated to versions compatible with 78. Google Provider, GContactSync and QuickFolders are not available in Add-ons.

Question: How can I get TB to recognize the abook-x.sqlite files? Do I need to add them manually to a config file somewhere? Or do I need to recreate them, find out the file names they are using, delete those files and rename my existing files to match? (Why is everything SO HARD WITH THUNDERBIRD?)

Question: Why is it so hard to get from the extension page in Options to the https://addons.thunderbird.net page for that extension? There should be a link there, always!

Question: Why does https://addons.thunderbird.net always insist on giving me the latest version? And force me to manually find the previous version. It should be so simple to pass the TB version number from TB to the page when it is launched from Options. (Oh, yeah, you want to make it as hard as possible for us to stayu on older versions, don't you.)

sqlite file names that are used by TB are based on what is specified in "pref.js" file , that file is inside the TB-"profile" folder ("xxxxxxxx.<profilename>"). i dont know if there is a manual command in TB that can convert old mab format into the new format sqlite, exists or not , wait for other supporters to answer . i know new TB does this conversion only on update/upgrade time. TB does not rename, it converts format of mab, into sqlite.

in addon download button, user have to right-click to bring properties popup > then "Save as".

and please wait for other better answers to your Questions.

EDIT: copy your old-profile , rename all BAK files "filename.file-extension.bak" into "filename.file-extension" (so "abook.mab.bak" needs to be renamed into "abook.mab" . then use this old-profile (or specify this old-profile via commandline to a newer v78 series TB) , then v78 series TB will convert the "abook.mab" into "abook.sqlite" & rename the "abook.mab" into "abook.mab.bak" , so v78 does this auto conversion during when it starts with a old-profile first-time.