To succesfully build with clang on ppc64le you need both https://bitbucket.org/jpommier/pffft/pull-requests/2 and https://bitbucket.org/jpommier/pffft/pull-requests/3, see https://bugzilla.mozilla.org/show_bug.cgi?id=1738872
Unfortunately the resulting binary gives me the following message at startup: "The bookmarks and history system will not be functional because one of Firefox's files is in use by another application".
I've tried to remove both ~/.mozilla and ~/.cache/mozilla but it didn't help. I've previously built it with gcc and it worked fine (it didn't need the before-mentioned patches).
Upstream issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1757271