Thunderbird is built on the Mozilla platform, the same base that Firefox is built from. As such the two projects share a lot of code and much of the documentation for one will apply, in many ways, to the other. If at any point you are looking for answers that you can't find here, try looking at Mozilla's Developer site called: MDN.
Mozilla uses the Mercurial version control software to propose, review, incorporate, and log changes to its code. In order to contribute to Thunderbird, you will need to be able to use this software.
Information for how to install Mercurial is available via the download page on their wiki.
Get the latest Mozilla source code from Mozilla's
mozilla-central Mercurial code repository, and check it out into a local directory
source (or however you want to call it). Then, get the latest Thunderbird source code from Mozilla's
comm-central Mercurial code repository. It now needs to be placed inside the Mozilla source code, in a directory named
comm/ (this is inverse from Thunderbird 59 and earlier):
hg clone https://hg.mozilla.org/mozilla-central source/cd source/hg clone https://hg.mozilla.org/comm-central comm/
Mozilla-central will build Firefox without the comm-central repo present and a few options set (detailed on the Building Thunderbird page). Mozilla-central is the Firefox codebase and comm-central features the additions that turn Firefox into Thunderbird.
Let's build the latest Thunderbird: