It's not us, it's Whatsapp. They're now using an asm.js version
of the Curve25519 key exchange system, and asm.js based scripts
don't work properly on PowerPC because of different endianness.
They used to use a different, more system-generic polyfill, which
did work (I remember using it), but they don't anymore. If you
downgrade to 38.3, you'll find it still doesn't work anymore.
There is no quick solution to this on the horizon. Optimally
when Firefox implements Curve25519 internally, then we can just use
that, which should "just work" when Whatsapp changes their code to
support it. However, Firefox doesn't support this yet, and may not
for some time. See https://bugzilla.mozilla.org/show_bug.cgi?id=957105