This file is indexed.

prerm is in firefox-esr 52.8.1esr-1~deb8u1.

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.

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#!/bin/sh -e

if [ "$1" = "remove" ] || [ "$1" = "deconfigure" ] ; then
    update-alternatives --remove x-www-browser /usr/bin/firefox-esr
    update-alternatives --remove gnome-www-browser /usr/bin/firefox-esr
fi

if [ "$1" = "remove" ]; then
    rm -rf /usr/lib/firefox-esr/updates
fi