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);