Trixie
6.12.88+deb13-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.12.88-1 (2026-05-15) x86_64 GNU/Linux


Python 3.13 is only one Trixie's official version,
but Thunderbird building process depends on 3.12 (or lower)
(sample of error message is attached below)

It may not match with beauty of Debian a bit,
but maybe it was too difficult to keep Python 3.12 in Trixie.

Firefox-esr also originally depends on 3.12 but can build
in the current stable environment probably with some/lot of hacks like

debian/rules:   MOZ_OBJDIR=debian/objdir MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE=none $(PYTHON) ./mach python --virtualenv build -c ""

and other patches/efforts. (I am sorry if I am wrong)

I cannot generate such patches for Thunderbird but at least I can make this report as recognized issue and show how to make successful build.

1. Get and make Python 3.12 locally

$ wget https://www.python.org/ftp/python/3.12.13/Python-3.12.13.tar.xz
$ ./configure --enable-optimizations
$ make -j$(nproc)
$ sudo make altinstall

2. Use local 3.12 as python3 and check

$ ln -s /usr/local/bin/python3.12 ~/bin/python3
$ export PATH=~/bin:$PATH
$ python3 --version

3. Build (just summary/example):
$ apt-get source thunderbird
then, in a proper directory
$ dpkg-buildpackage -b -us -uc -j1

4. After the build Thunderbird, you may remove ~/bin/python3

Happy build!  Taro


*** error message ***

Exception: According to "pip check", the current Python environment has package-compatibility issues.
---
Note that you are running Mach with Python 3.13.5, which is higher
than the highest known working version of Python for Mach. Consider
running Mach with Python 3.12 or lower.
make[2]: *** [debian/rules:139: override_dh_auto_configure] Error 1
make[2]: Leaving directory '/.../thunderbird-140.11.0esr'

***