The source code is here: https://hg.mozilla.org/comm-central https://hg.mozilla.org/comm-central, although you also need a checkout of https://hg.mozilla.org/mo

The source code is here: https://hg.mozilla.org/comm-central https://hg.mozilla.org/comm-central, although you also need a checkout of https://hg.mozilla.org/mozilla-central https://hg.mozilla.org/mozilla-central for the Gecko code. Also, Thunderbird does support FreeBSD, in that FreeBSD-specific bugs will be triaged and fixed, although it's obviously not a primary platform of support.