build instructions do not work as stated in "getting started"

Hi. I am trying to get going with thunderbird development, but although following the getting started the build fails.

These are the instructions I am following:
https://developer.thunderbird.net/thunderbird-development/building-thunderbird

getting mercurial, source checkout and setting up prerequisites works fine.
Without a custom mozconfig I am also able to successfully build firefox, but as soon as I add mozconfig, it breaks.

The error I get: TypeError: 'NoneType' object is not subscriptable
More log: https://pastebin.com/ZPUv5PW0

This is the content of my mozconfig as recommended in the getting started:

ac_add_options --enable-project=comm/mail
ac_add_options --enable-debug

When reading further links on your documentation I discovered several broken links:
https://developer.mozilla.org/en-US/Configuring_Build_Options
https://developer.mozilla.org/en-US/docs/Mozilla/Developer_guide/mach

Are these docs out of date. If so, where can I find up to date ones?
How do I get started with thunderbird development?

Thanks.

Start with the getting-started build page and the reported mozconfig containing --enable-project=comm/mail and --enable-debug; compare it with the linked build log. Check the two broken documentation links and verify the instructions against a Thunderbird build. Done means the documented build steps and links are current and reproducible.