Comment 20 for bug 1819013

I'm back with a workaround (i know, it is not elegant)

I copied the libstdc++ library from IBM toolchain (see my first post to find the url) to /usr/lib/powerpc64le-linux-gnu and made the symlink.

Now I have in /usr/lib/powerpc64le-linux-gnu :
libstdc++.so.6.0.25
and
libstdc++.so.6 -> libstdc++.so.6.0.25 (The symlink)

Firefox 66.0.2+build1-0ubuntu0.16.04.1 (update version) works fine.

lsb_releas -a
Distributor ID: Ubuntu
Description: Ubuntu 16.04.6 LTS
Release: 16.04
Codename: xenial
uname -a

Linux testubu16 4.4.0-143-generic #169-Ubuntu SMP Thu Feb 7 07:56:40 UTC 2019 ppc64le ppc64le ppc64le GNU/Linux