How to develop the best mail client in existence.

Getting Started

Read the Developer Guide on MDN. That will give you a general overview of the Mozilla ecosystem. Thunderbird participates in that ecosystem, so you will need that overview as you join in the work.

Read the Simple Thunderbird build guide on MDN to learn how to run your first build successfully. You will learn how to get the code, set any necessary configuration, build the calendar, rebuild after checking out new code, etc. It's a good read. Don't just skim it.


Code is located on Mozilla's Mercurial server.

There is also some code hosted on GitHub.

  • Thundernest contains the code for (this site), as well as the Thunderbird web server setup scripts.
  • Thunderbird on GitHub contains several Thunderbird related repositories.


All Thunderbird bugs live on Mozilla's Bugzilla. Bugzilla is a powerful tool and can be intimidating if you are not used to it. So check out these pre-defined searches to test the waters:

  • You can help out by triaging incoming bugs. Known bugs should be marked as duplicates, unconfirmed bugs often needs someone to see if they too can reproduce.
  • "Good first bugs" which are extra easy to try and fix when you are just starting out.
  • Mentored Bugs have a mentor who commits to helping you every step of the way.
  • Thunderbird bug queries is a wiki page with lots of useful searches.

Extension/Add-On Development

See the documentation on MDN.


Make Thunderbird easy to use, and easy on the eyes.

Please share ideas and concepts by posting them on Bugzilla. You can also discuss design in the #maildev irc channel, or the Discourse category.

Style Guide

  • Looking for the Thunderbird logo, icons, color palette, or other design resources? Check out our style guide.


Help make sure the answers are written down before the questions are asked.

Where documentation lives

Contributing to documentation


Make sure the entire world can use Thunderbird.

Quality Assurance

Help catch those bugs before they get loose.

See Thunderbird:Testing on the Mozilla Wiki.


Here are the places and ways Thunderbird communicates.

Discussion Forums

Live Chat

IRC Channels on

You can use if you don't already have a preferred IRC client.

  • #maildev - For hacking on Thunderbird.
  • #extdev - For building Thunderbird add-ons/extensions.
  • #thunderbird - For general questions and support.
  • #tb-qa - For QA chat.

Mailing Lists

Also check the Communication Channels wiki page.