From someone who complain abouy FreeBSD haters you seem very little informed on what is FreeBSD, how the project is organized and how people typically install software on it.
> It's disappointing for that there is no FreeBSD release nor any easy way to find the source for that matter.
The point I'm making their were no packages for FreeBSD nor any easy listing of accessing the source.
The GitHub repo I found, did not contain nor was I aware that Mozilla uses Mercurial. Precompiled packages exist for Linux, but not FreeBSD outside of the FreeBSD port system. Not even a mention.
I agree the information about the location of the source code is buried in the developer doc that you access when hitting contribute. I think an open source product could do better and make the source code more easily accessible.
https://www.freshports.org/mail/thunderbird/