I have found this to be successful:

 https://support.mozilla.org/en-US/questions/1438626

Thanks for your helpful response. I think I've finally got Thunderbird to stop updating itself. I have:

(1) [Folder in which Thunderbird is located]\ distribution\"policies.json" containing: {

 "policies": {
  "DisableAppUpdate": true,
  "DisableFeedbackCommands": true,
  "DisableSystemAddonUpdate": true,
  "DisableTelemetry": true,
  "ExtensionUpdate": false,
  "ManualAppUpdateOnly": true,
  "AppAutoUpdate": false,
  "AppUpdateURL": "https://yoursiteisnothere.com"  
 }

} (The first four settings are from other suggestions people or chatbots made; the last three are yours.) Unfortunately going to Settings>General > Updates does not say “Failed to check for updates” , but only the usual "Updates disabled by your system administrator", so I can't verify.

In addition I have:

(2) HKEY_LOCAL_MACHINE\Software\Policies\Mozilla\Thunderbird: DisableAppUpdate = 1.

(3) C:\ProgramData\Mozilla\updates\308...39CB\update-config.json, which contains HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Mozilla\Thunderbird\DisableAppUpdate = 1

(4) [Thunderbird Profile folder]\prefs.js, which contains: user_pref("app.update.auto", false); user_pref("app.update.auto.migrated", true);

(5) C:\ProgramData\Mozilla\updates\308046B0AF4A39CB\update-config.json, which contains: {"app.update.auto":false}

I find it disturbing that stopping Thunderbird automatic updates requires so many different steps, and missing any one of them could cause the process to fail.

Again, thanks for the help!

I thought the problem was solved: after two days and several reboots of my computer Thunderbird had stayed with the version I wanted. However when I opened it the next day I found it had updated and the folder called 'distribution' I'd put in the folder containing thunderbird.exe was gone! Once again I restored to a backup I'd made. This time I made policies.json read-only.

The next day when I turned on my computer, Thunderbird opened in the version I wanted. However later that day I closed it momentarily and found when I reopened it, it had updated again and the folder 'distribution" and file policies.json were gone - despite my having made policies.json read-only! And not only that, there was a new folder called 'tobedeleted' in its place! I found 'tobedeleted' contained 16 binary files, each of a couple hundred MB, with names like 'nsb6FCF.tmp'. Once again, I restored everything and rebooted. I verified that all references (see below) were correct, then opened Thunderbird. It had updated again!

What is going on?? How can I keep Thunderbird from updating, short of giving up and moving the Chrome?

Here are all the procedures I have in place in my fruitless attempts to prevent Thunderbird from updating.

(1) In the folder in which Thunderbird.exe is located there is a folder 'distribution' containing file 'policies.json' containing: {

 "policies": {
  "DisableAppUpdate": true,
  "DisableFeedbackCommands": true,
  "DisableSystemAddonUpdate": true,
  "DisableTelemetry": true,
  "ExtensionUpdate": false,
  "ManualAppUpdateOnly": true,
  "AppAutoUpdate": false,
  "AppUpdateURL": "https://yoursiteisnothere.com"  
 }

}

(2) C:\ProgramData\Mozilla\updates\308046B0AF4A39CB\update-config.json contains {"app.update.auto":false}

(3) HKEY_LOCAL_MACHINE\Software\Policies\Mozilla\Thunderbird: DisableAppUpdate = 1 .

(4) In Thunderbird's Profile folder there is a file prefs.js which contains: user_pref("app.update.auto", false); user_pref("app.update.auto.migrated", true);

After 5 days and >5 reboots with no unauthorized updates, I think the problem is finally solved. My conclusion from all this is that there are many places where you have to turn off automatic updates, and if you miss even one of them Thunderbird will automatically update. See my reply above for 4 of those places. I've since found two more places:

'C:\ProgramData\Mozilla-1de4ee...\updates\F2C374...\update-config.json' had the following line:

     {"app.update.auto":true}

which I changed to "false".

'C:\ProgramData\Mozilla-1de4ee...\updates\BE45EB...\update-config.json' had the following line:

  {"__DEFAULTS__":{"app.update.auto":false,"app.update.background.enabled":false},"app.update.auto":false}

which was already set to "false".

With these 6 places all saying "false" it appears I've finally got them all. This leaves the question "Why doesn't Thunderbird have one and only one place where auto updates is turned on or off?"

What you have described is unusual, so I wonder if you may have a program running that autoupdates other software. One such product is Glary Utilities, and I've heard that Norton antivirus (?) may do that. BUT, I am not a Thunderbird developer. I have several versions of Thunderbird (esr, monthly, beta, daily) and the ones I've specified to not autoupdate are behaving as intended. I wish I could provide more assistance.

Interesting speculation! No, I don't have Glary Utilities or Norton antivirus on my computer. I'm sure a number of programs that I do have are set to auto-update, but only for that specific program. In any case, my whack-a-mole search seems to have found everything and Thunderbird is behaving itself now. Thanks for the help. (BTW, the reason I'm reluctant to update is that I have a couple of add-ons that I use all the time and which have stored a lot of data that I would lose if I updated, since they aren't compatible with later versions of Thunderbird.)

It must be a Windows thing as I'm on Linux and have only had to make a policies.json file containing just "DisableAppUpdate": true.