postrm is in thunderbird 1:45.8.0+build1-0ubuntu0.12.04.1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | #!/bin/sh
set -e
MOZ_PKG_NAME=thunderbird
abort_mv_conffile() {
    local CONFFILE="$1"
    if [ -e "$CONFFILE.dpkg-remove" ]; then
        echo "Reinstalling $CONFFILE that was moved away"
        mv "$CONFFILE.dpkg-remove" "$CONFFILE"
    fi
}
if [ "$1" = "abort-install" -o "$1" = "abort-upgrade" ] ; then
    abort_mv_conffile "/etc/${MOZ_PKG_NAME}/pref/thunderbird.js"
fi
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