I have an older Intel Atom laptop on which I've recently installed the i386 version of Debian 9.3.1. I'm running it headless, and have no need for Firefox. Much to my surprise, when I did,
sudo apt-get remove firefox-esr
the system removed firefox and installed thunderbird (and also lightning). I then did,
sudo apt-get remove lightning sudo apt-get remove thunderbird
and the second command above removed thunderbird as expected, and reinstalled firefox-esr.
Is there some way to not have both of these applications? Perhaps there is some dependency elsewhere that can be satisfied by either firefox or thunderbird, thus removing one installs the other?