postrm is in firefox 28.0+build2-0ubuntu2.
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 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60  | #!/bin/sh
set -e
MOZ_PKG_NAME=firefox
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
}
abort_rm_conffile() {
    local CONFFILE="$1"
    if [ -e "$CONFFILE.dpkg-remove" ]; then
        echo "Reinstalling $CONFFILE that was moved away"
        mv "$CONFFILE.dpkg-remove" "$CONFFILE"
    fi
    if [ -e "$CONFFILE.dpkg-backup" ]; then
        echo "Reinstalling $CONFFILE that was backupped"
        mv "$CONFFILE.dpkg-backup" "$CONFFILE"
    fi
}
purge_conffile() {
    local CONFFILE="$1"
    rm -f "$CONFFILE.dpkg-bak" "$CONFFILE.dpkg-remove" "$CONFFILE.dpkg-backup" || true
}
if [ "$1" = "purge" ]; then
    APP_PROFILE="usr.bin.firefox"
    rm -f /etc/apparmor.d/force-complain/$APP_PROFILE || true
    rm -f /etc/apparmor.d/disable/$APP_PROFILE || true
    rm -f /etc/apparmor.d/local/$APP_PROFILE || true
    rm -f "/etc/apparmor.d/abstractions/ubuntu-browsers.d/firefox" || true
    rmdir /etc/apparmor.d/local 2>/dev/null || true
    purge_conffile "/etc/${MOZ_PKG_NAME}/profile/bookmarks.html"
    purge_conffile "/etc/${MOZ_PKG_NAME}/profile/localstore.rdf"
    purge_conffile "/etc/${MOZ_PKG_NAME}/profile/mimeTypes.rdf"
    purge_conffile "/etc/${MOZ_PKG_NAME}/profile/prefs.js"
    purge_conffile "/etc/${MOZ_PKG_NAME}/profile/chrome/userChrome-example.css"
    purge_conffile "/etc/${MOZ_PKG_NAME}/profile/chrome/userContent-example.css"
fi
if [ "$1" = "abort-install" -o "$1" = "abort-upgrade" ] ; then
    abort_rm_conffile "/etc/${MOZ_PKG_NAME}/profile/bookmarks.html"
    abort_rm_conffile "/etc/${MOZ_PKG_NAME}/profile/localstore.rdf"
    abort_rm_conffile "/etc/${MOZ_PKG_NAME}/profile/mimeTypes.rdf"
    abort_rm_conffile "/etc/${MOZ_PKG_NAME}/profile/prefs.js"
    abort_rm_conffile "/etc/${MOZ_PKG_NAME}/profile/chrome/userChrome-example.css"
    abort_rm_conffile "/etc/${MOZ_PKG_NAME}/profile/chrome/userContent-example.css"
    abort_mv_conffile "/etc/${MOZ_PKG_NAME}/pref/firefox.js"
fi
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