Thunderbird uses Mozilla's Bugzilla platform to report and track bugs. The site can also be used to generate enhancement bugs, which can be used for feature requests. If you want to become a contributor to Thunderbird, you will need an account on Bugzilla as you will submit patches through this platform.
If you want to contribute to the Thunderbird website, this documentation, or addons.thunderbird.net - you can find those repositories and their issue trackers on the Thunderbird GitHub page. You'll need a GitHub account to contribute there.
Getting Plugged into the Community
We have a complete listing of the ways in which you can get involved with Thunderbird on our website. Below are some quick references from that page that you can use if you are looking to contribute to Thunderbird core right away.
If you want to participate in discussions about Thunderbird development, these are the main channels.
TB-Planning: This mailing list is higher level topics like: the future of Thunderbird, potential features, and changes that you would like to see happen. It is also used to discuss a variety of broader issues around community and governance of the project.
Topicbox: A moderated mailing list for discussing engineering plans for Thunderbird. It is a place where you can raise questions and ideas for core Thunderbird development.
Add-on Developers: A list for Thunderbird add-on developers and aspiring add-on developers to ask questions and share knowledge.
If you want to ask questions in real-time about how to hack on Thunderbird, you can join our development chat channel at #maildev:mozilla.org