Markdown Paste for Thunderbird

Paste GitHub-Flavored Markdown into a Thunderbird HTML compose window and see it rendered instantly as formatted rich text — tables, code blocks with syntax highlighting, task lists, strikethrough, and more, styled to closely match how GitHub renders Markdown.

Features

• Live inline paste rendering — paste Markdown text anywhere in the compose editor and it is immediately converted to formatted HTML in place.
• Toggle button — a toolbar button (and Ctrl+Alt+M) renders or un-renders the entire draft on demand, for drafts typed directly as Markdown rather than pasted.
• GitHub Flavored Markdown (GFM): headings, bold/italic, lists, links, blockquotes, tables, ~~strikethrough~~, task lists (- [ ] / - [x]), autolinked URLs, and fenced code blocks with syntax highlighting (via highlight.js, GitHub theme).
• Safe by design — all rendered HTML is sanitized with DOMPurify before insertion; no eval(), no remote code, fully CSP-compliant, no network access needed.

How it works

• A background script registers a compose script (messenger.composeScripts.register()) that runs inside the live DOM of every compose window.
• The compose script listens for native paste events. Pasted plain text is converted to HTML with marked.js (configured with { gfm: true, breaks: true } to match GitHub's own rendering behavior), sanitized, and inserted at the cursor using the Selection/Range API (cursor position is preserved after insertion).
• The compose toolbar button (compose_action) sends a message to the active compose tab to toggle full-draft rendering. The original Markdown source is preserved in a hidden attribute so the draft can be converted back to source text.

This add-on targets recent Thunderbird versions only (140+) and does not attempt to support older releases.

No Experiment APIs, no legacy XPCOM access, and no clipboardRead permission are required — clipboard text is read synchronously from the native paste event, and the rest is standard modern WebExtension APIs.

Installation (local / manual)

1. Download or clone this repository.

2. In Thunderbird: Menu → Add-ons and Themes → gear icon → Debug Add-ons.

3. Click Load Temporary Add-on… and select manifest.json from this folder.

4. Open a new compose window, paste some Markdown, and it should render immediately. Use the toolbar button or Ctrl+Alt+M to toggle rendering of the whole draft.

   Temporary add-ons are removed when Thunderbird restarts. To install permanently, package it as an .xpi (see below) and use Install Add-on From File… instead — Thunderbird does not require extensions to be signed, so this works out of the box.

Packaging as a permanent .xpi

Run package.ps1 (Windows PowerShell) from the project root:

./package.ps1

This produces markdown-paste.xpi in the project root, ready for Install Add-on From File…, or for submission to addons.thunderbird.net (see ATN_SUBMISSION_GUIDE.md).

Development

• Lint with web-ext:

  npx web-ext lint --source-dir . --ignore-files vendor/** icons/**

• Compose script console: while the add-on is loaded temporarily, click the Inspect link next to it in Debug Add-ons, open a compose window, and use the DevTools console attached to that window to see logs/errors from composescript.js.

Running the automated test suite

The tests/ folder and its jsdom devDependency are dev-only — they are not required to build or run the extension itself (the shipped .xpi has zero Node.js dependencies, see BUILD.md), and package.ps1 never includes tests/, node_modules/, package.json, or package-lock.json in the packaged .xpi.

npm install
npm test

The tests load the actual background.js/composescript.js files (via jsdom + a stubbed messenger API) rather than reimplementing their logic, so they exercise the real paste-rendering, whole-draft toggle, and signature-exclusion behavior end-to-end. See tests/ for coverage of: single-line vs. multi-line vs. block-level Markdown paste rendering, render/un-render round-trips, Thunderbird signature exclusion, plain-text compose mode, composestyles.css scoping, manifest.json validity, and background.js message wiring.

Project structure

manifest.json          Extension manifest (MV2, MailExtension)
background.js          Registers the compose script, handles the toolbar button & command
composescript.js       Runs inside the compose editor DOM: paste interception + toggle render
composestyles.css      GitHub-like styling for rendered Markdown output
vendor/                 Bundled third-party libraries (marked.js, DOMPurify, highlight.js)
icons/                  Toolbar/listing icons
tests/                  Automated test suite (dev-only, see "Running the automated test suite")
package.ps1             Helper script to zip the extension into an .xpi
ATN_SUBMISSION_GUIDE.md  Steps + listing text for submitting to addons.thunderbird.net

License

MIT — see LICENSE.