(cherry picked from commit 63e8aa5)

This is just a minimalist approach to make newer Firefox build.
I looked at diff to master and picked differences that seemed required.

In particular, some of llvm parts weren't picked, as I expected they
would make it harder for me to be sure I haven't broken anything else,
and they only appeared useful on Clang platforms (no Firefox there).

Again, firefox-esr doesn't even get rebuilt.

This version switching is a bit inconsistent with the way we do it
for NSS, but somehow... it felt better (?) to write this way.
Anyway, I don't mind if someone improves it; I mainly wanted to
fix Firefox quickly with minimal risk of breaking other stuff.