Firefox 78 is released and is running on this Talos II. This version in particular features an updated RegExp engine but is most notable (notorious) for disabling TLS 1.0/1.1 by default (only 1.2/1.3). Unfortunately, because of craziness at $DAYJOB and the lack of a build waterfall or some sort of continuous integration for ppc64le, a build failure slipped through into release but fortunately only in the (optional) tests. The fix is trivial, another compilation bug in the profiler that periodically plagues unsupported platforms, and I have pushed it upstream in bug 1649653. You can either apply that bug to your tree or add ac_add_options --disable-tests to your .mozconfig. Speaking of, as usual, the .mozconfigs we use for debug and optimized builds have been stable since Firefox 67.

UPDATE: The patch has landed on release, beta and ESR 78, so you should be able to build straight from source.

https://www.talospace.com/2020/06/firefox-78-on-power.html