postinst is in xul-ext-websites-integration 2.3.6+13.10.20130920.1-0ubuntu1.2.
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 | #!/bin/sh
set -e
echo "Please restart all running instances of firefox, or you will experience problems."
if [ -d /var/run ] ; then
touch /var/run/firefox-restart-required
fi
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