This file is indexed.

prerm is in firefox 45.0.2+build1-0ubuntu1.

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

set -e

MOZ_APP_NAME=firefox

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