Comment 11 for bug 1517884

If you don't want to change your repositories (even temporarily), I have created a PPA just for that package here:

https://launchpad.net/~zeehio/+archive/ubuntu/libxp

I have published the package for xenial:

sudo add-apt-repository ppa:zeehio/libxp
sudo apt-get update
sudo apt-get install libxp6

It contains the latest version from Debian before it was removed.