1. Mercurial is the version management system that is used by Mozilla and it is necessary. (in my case: sudo pacman -S mercurial)
   • > hg version
Mercurial Distributed SCM (version 6.9)
(see https://mercurial-scm.org for more information)

Copyright (C) 2005-2024 Olivia Mackall and others
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
2. Make the working directory with: mkdir tb-build && cd tb-build
3. wget https://hg.mozilla.org/comm-central/raw-file/tip/python/rocboot/bin/bootstrap.py (downloads the Python bootstrap script)
4. chmod +x bootstrap.py (gives executable file permissions to the script)
   • > ls -l
total 16
-rwxr-xr-x 1 chris chris 16025 Jan 17 18:13 bootstrap.py
drwxr-xr-x 1 chris chris  2264 Jan 17 18:14 mozilla-unified

5. ./bootstrap.py (actually runs the script). The first problem I encountered was the rustup version. It was solved with rustup install stable and rustup default stable
6. Build the sources with ./mach build and then run with ./mach run

The whole building process lasted approximately 8 minutes on my AMD Ryzen 9 7950X3D 16-Core Processor and required 30.1 GiB (32,355,790,419), 1,401,719 files, 138,437 sub-folders

More details at https://developer.thunderbird.net/thunderbird-development/building-thunderbird