I would like to install Firefox v52 ESR on Ubuntu 16.04 in order to access a legacy Java applet. The current version Firefox 60 ESR no longer allows me to do that.
In effect, I would like to
apt-get install firefox-esr=52.5.3esr+build1-0ubuntu0.16.04.1 from
deb http://ppa.launchpad.net/mozillateam/ppa/ubuntu xenial main, but the version is apparently no longer available from this PPA.
How can I get hold of an older version of an Ubuntu package in a case like this?