Thunderbird MailExtension Development Skill

Thunderbird MailExtension Development

Complete reference for building, testing, and publishing email extensions for Mozilla Thunderbird.

Overview

Thunderbird extensions use the MailExtension API (based on WebExtensions) with the messenger.* namespace. Thunderbird supports both Manifest V2 and V3 since version 128.

Key Characteristics:

Global namespace: messenger (Thunderbird-specific) + browser (standard WebExtensions)
Both MV2 and MV3 supported (Thunderbird 128+)
Thunderbird-specific APIs: accounts, addressBooks, compose, folders, mailTabs, messages, messageDisplay
Submission via ATN (addons.thunderbird.net)

Version Requirements

| Version | Status | Notes | |---------|--------|-------| | 128.x (ESR) | Current | Full MV2 + MV3 support | | 115.x | Legacy | End of support | | < 115 | Deprecated | Not recommended |

Best Practice: Set strict_min_version to "128.0"

Manifest Structure

Manifest V3 (Recommended, Thunderbird 128+)

{
  "manifest_version": 3,
  "name": "My Thunderbird Extension",
  "version": "1.0.0",
  "description": "Extension description",
  "author": "Your Name",

  "browser_specific_settings": {
    "gecko": {
      "id": "extension@example.com",
      "strict_min_version": "128.0"
    }
  },

  "icons": {
    "16": "icons/icon-16.png",
    "32": "icons/icon-32.png",
    "64": "icons/icon-64.png"
  },

  "background": {
    "service_worker": "background.js",
    "type": "module"
  },

  "action": {
    "default_popup": "popup.html",
    "default_title": "My Extension",
    "default_icon": "icons/icon-32.png"
  },

  "permissions": [
    "storage",
    "messagesRead",
    "addressBooks"
  ]
}

Manifest V2 (Still Supported)

{
  "manifest_version": 2,
  "name": "My Thunderbird Extension",
  "version": "1.0.0",
  "author": "Your Name",

  "browser_specific_settings": {
    "gecko": {
      "id": "extension@example.com",
      "strict_min_version": "128.0"
    }
  },

  "background": {
    "scripts": ["background.js"],
    "type": "module"
  },

  "browser_action": {
    "default_popup": "popup.html",
    "default_title": "My Extension"
  },

  "permissions": [
    "storage",
    "messagesRead",
    "addressBooks"
  ]
}

MV2 vs MV3 Key Differences

| Feature | MV2 | MV3 | |---------|-----|-----| | Toolbar button | browser_action | action | | Background | background.scripts | background.service_worker | | Execute script | tabs.executeScript | messenger.scripting.messageDisplay.executeScript | | Compose scripts | composeScripts | scripting.compose | | Contacts API | messenger.contacts.* | messenger.addressBooks.contacts.* (vCard only) |

All Manifest Keys Reference

Metadata:

name (required) - Extension name
version (required) - Version string
description - Short description
author - Author name
icons - Extension icons

Thunderbird-Specific:

browser_specific_settings.gecko.id - Required for ATN
browser_specific_settings.gecko.strict_min_version - Minimum version

Background & Scripts:

background - Service worker (MV3) or scripts (MV2)
message_display_scripts (MV2) - Scripts for displayed messages

UI Components:

action (MV3) / browser_action (MV2) - Main toolbar button
compose_action - Compose window toolbar button
message_display_action - Message view toolbar button

Permissions:

permissions - API permissions
experiment_apis - Custom Experiment APIs

Other:

commands - Keyboard shortcuts
options_ui - Options page

UI Actions (Toolbar Buttons)

Main Toolbar (action / browser_action)

{
  "action": {
    "default_popup": "popup.html",
    "default_title": "My Extension",
    "default_icon": {
      "16": "icons/icon-16.png",
      "32": "icons/icon-32.png"
    }
  }
}

// Listen for clicks (if no popup)
messenger.action.onClicked.addListener((tab) => {
  console.log("Action clicked");
});

// Update badge
await messenger.action.setBadgeText({ text: "5" });
await messenger.action.setBadgeBackgroundColor({ color: "#ff0000" });

// Update icon
await messenger.action.setIcon({ path: "icons/icon-active.png" });

Compose Window (compose_action)

{
  "compose_action": {
    "default_popup": "compose_popup.html",
    "default_title": "Compose Tool",
    "default_icon": "icons/compose-icon.png"
  }
}

// Listen for clicks in compose window
messenger.composeAction.onClicked.addListener((tab) => {
  const details = await messenger.compose.getComposeDetails(tab.id);
  console.log("Compose action clicked:", details.subject);
});

Message Display (message_display_action)

{
  "message_display_action": {
    "default_popup": "message_popup.html",
    "default_title": "Message Tool",
    "default_icon": "icons/message-icon.png"
  }
}

// Listen for clicks on message
messenger.messageDisplayAction.onClicked.addListener(async (tab) => {
  const message = await messenger.messageDisplay.getDisplayedMessage(tab.id);
  console.log("Message action clicked:", message.subject);
});

Message Display Scripts

MV2 Configuration

{
  "message_display_scripts": [
    {
      "matches": ["<all_urls>"],
      "js": ["message_content.js"],
      "css": ["message_styles.css"]
    }
  ]
}

MV3 Configuration

// In background.js
await messenger.scripting.messageDisplay.executeScript({
  tabId: tabId,
  files: ["message_content.js"]
});

Available APIs in Display Scripts

Limited APIs available:

messenger.runtime.connect(), messenger.runtime.sendMessage()
messenger.runtime.onConnect, messenger.runtime.onMessage
messenger.i18n.getMessage(), messenger.i18n.getAcceptLanguages()
messenger.storage.*

// message_content.js
// Send message to background
const response = await messenger.runtime.sendMessage({
  action: "processMessage",
  content: document.body.innerText
});

// Listen for messages from background
messenger.runtime.onMessage.addListener((message, sender, sendResponse) => {
  if (message.action === "highlight") {
    // Highlight text in message
    document.body.innerHTML = document.body.innerHTML.replace(
      message.text,
      `<mark>${message.text}</mark>`
    );
  }
});

Deep Dives

For detailed reference on specific topics, load these reference files on demand:

Messenger APIs - Complete messenger.* API reference (accounts, messages, folders, addressBooks, compose, mailTabs): ./references/messenger-apis.md
Experiment APIs - Custom Experiment API development guide: ./references/experiment-apis.md
ATN Submission - Publishing workflow and review criteria: ./references/atn-submission.md
Testing & Migration - Debugging, testing workflow, and legacy migration: ./references/testing-migration.md

Best Practices

Code Organization

my-extension/
├── manifest.json
├── background.js
├── popup.html
├── popup.js
├── compose_popup.html
├── compose_popup.js
├── api/
│   └── myapi/
│       ├── schema.json
│       └── implementation.js
├── icons/
│   ├── icon-16.png
│   ├── icon-32.png
│   └── icon-64.png
├── _locales/
│   ├── en/
│   │   └── messages.json
│   └── it/
│       └── messages.json
└── README.md

Error Handling

async function safeAsync(fn) {
  try {
    return await fn();
  } catch (error) {
    console.error("Error:", error);
    return { error: error.message };
  }
}

// Usage
const result = await safeAsync(() => messenger.messages.get(messageId));
if (result.error) {
  console.error("Failed to get message:", result.error);
}

Performance

Use pagination for large message lists
Cache frequently accessed data
Debounce rapid events
Use messages.query() with filters instead of list() + filter manually

Security

Validate all user input
Sanitize HTML before display
Use minimal permissions
Don't store sensitive data in storage.local unencrypted
Validate message content before processing

Differences from Firefox WebExtensions

| Feature | Firefox | Thunderbird | |---------|---------|-------------| | Namespace | browser.* | messenger.* (mail) + browser.* (common) | | Context | Web browser | Email client | | Content scripts | Work on web pages | Only in web tabs, not email content | | Main action | browser_action / action | Same + compose_action, message_display_action | | Mail APIs | None | accounts, compose, messages, etc. | | Experiments | Limited | Common for email-specific features | | Store | AMO | ATN |

File Structure Template

my-thunderbird-extension/
├── manifest.json
├── background.js
├── popup.html
├── popup.js
├── compose_popup.html
├── compose_popup.js
├── message_popup.html
├── message_popup.js
├── message_content.js
├── styles/
│   └── popup.css
├── icons/
│   ├── icon-16.png
│   ├── icon-32.png
│   └── icon-64.png
├── api/
│   └── myapi/
│       ├── schema.json
│       └── implementation.js
├── _locales/
│   ├── en/
│   │   └── messages.json
│   └── it/
│       └── messages.json
└── README.md

Quick Reference

Essential Permissions

{
  "permissions": [
    "storage",           // Data storage
    "messagesRead",      // Read messages
    "messagesMove",      // Move/copy/delete messages
    "addressBooks",      // Access contacts
    "compose",           // Compose windows
    "accountsRead",      // Read accounts
    "accountsFolders"    // Access folders
  ]
}

Essential APIs

// Messages
messenger.messages.list(folderId)
messenger.messages.get(messageId)
messenger.messages.query({ from, unread })
messenger.messages.update(messageId, { read: true })

// Folders
messenger.folders.get(folderId)
messenger.folders.getSubFolders(account)

// Compose
messenger.compose.beginNew({ to, subject, body })
messenger.compose.getComposeDetails(tabId)

// Address Books
messenger.addressBooks.list()
messenger.addressBooks.contacts.create(addressBookId, { vCard })

// Display
messenger.messageDisplay.getDisplayedMessage(tabId)
messenger.messageDisplayAction.onClicked

Workflow Summary

Develop: Write code, load temporarily
Debug: Use Debug Add-ons → Inspect
Test: Test all functionality
Build: Create .zip with manifest and scripts
Submit: Upload to ATN
Review: Respond to reviewer feedback
Publish: Extension goes live

Agent Skills: Thunderbird MailExtension Development

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