I'm currently doing the same thing, just for Firefox on Linux. I asked a question here on this subject, and found the solution myself. Read it, maybe it will help you solve your problem: https://support.mozilla.org/ru/questions/1364066 Note that manifest.json must contain the extension ID. Run Thunderbird with the --jsconsole command line option to see if there are any errors loading the extension.

Here's more useful information I found: https://unix.stackexchange.com/questions/559653/install-thunderbird-extension-globally https://support.mozilla.org/en-US/questions/1130750

I'm currently doing the same thing, just for Firefox on Linux. I asked a question here on this subject, and found the solution myself. Read it, maybe it will help you solve your problem: https://support.mozilla.org/ru/questions/1364066 Note that manifest.json must contain the extension ID. Run Thunderbird with the --jsconsole command line option to see if there are any errors loading the extension.

Here's more useful information I found: https://unix.stackexchange.com/questions/559653/install-thunderbird-extension-globally https://support.mozilla.org/en-US/questions/1130750

Well, this at least gives me a direction to search. I already have a <setupfolder>/distribution/policies.json file that works fine (disabling automatic updates and some other stuff).

What I find in the linked posts suggests to place a

<setupfolder>/distribution/extensions/manifest.json

file with some extension related stuff inside. We should note that this is the same place where the xpi of my extension is placed, the same xpi that Thunderbird should read and add to extensions. Let us say that I want do provide Russian language pack to all users that will start Thunderbird. In this place I would have a file named

<setupfolder>/distribution/extensions/langpack-ru@thunderbird.mozilla.org

which already contains its corresponding json.

If I do this the jsconsole throws:

1642237885402 addons.xpi WARN Ignoring distribution: not an XPI: C:\Program Files\Mozilla\ThunderBird\distribution\extensions\manifest.json

which makes me think that Thunderbird only likes XPI files in this place (which also makes much sense). So probably this is a good step into the right direction, but still I am not able to provide any addon to all users :(

You can join the Thunderbird Enterprise mailing list on Topicbox to ask your question there. https://thunderbird.topicbox.com/groups/enterprise

Also see https://enterprise.thunderbird.net/

I would gladly do it. I even make recurring donations to Thunderbird!! But where shall I exactly ask this specific question?

Follow the Topicbox link above. https://support.mozilla.org/en-US/questions/1364317#answer-1474436

So I understand now that I shall "improve" my policies.json file to instruct ThunderBird to install specific addons automatically when a user starts the program. I have been struggling a couple of hours, and I still could not figure out how my policies.json file shall be. I have been searching around for examples, without finding anything. I am trying to provide 3 language packs: Russian, German and Italian. My policies.json file is down here, and for what I can understand it should be working (AppUpdate is disabled). Can anybody instruct me, what I am doing wrong?

1. # # # # # #

{

 "manifest_version": 2,
 "policies": {
   "DisableAppUpdate": true,
   "ExtensionUpdate": true,
   "ExtensionSettings": {
   "*": {
       "blocked_install_message": "This extension is NOT allowed by you System Administrator",
       "install_sources": ["about:addons","https://addons.thunderbird.net/"],
       "installation_mode": "allowed",
       "allowed_types": ["extension","theme","dictionary","locale"]
     },
     "langpack-it@thunderbird.mozilla.org": {
       "installation_mode": "force_installed",
       "install_url": "https://addons.thunderbird.net/thunderbird/downloads/latest/tb-langpack-it/addon-640746-latest.xpi"
     },
     "langpack-de@thunderbird.mozilla.org": {
       "installation_mode": "force_installed",
       "install_url": "https://addons.thunderbird.net/thunderbird/downloads/latest/tb-langpack-de/addon-640754-latest.xpi"
     },
     "langpack-ru@thunderbird.mozilla.org": {
       "installation_mode": "force_installed",
       "install_url": "https://addons.thunderbird.net/thunderbird/downloads/latest/tb-langpack-ru/addon-640736-latest.xpi"
     }
   }
 }

}