Firefox 63 vs. sendmsg()

[Jan Beich]

Can someone help debug https://bugzilla.mozilla.org/show_bug.cgi?id=1475970#c7 ?
I'm not familar with socket code.

Steps to reproduce:
  $ pkg install python27
  $ hash git 2>/dev/null || pkg install mercurial
  $ hg clone https://hg.mozilla.org/mozilla-unified firefox ||
    git clone https://github.com/mozilla/gecko-dev firefox
  $ cd firefox
  $ hg update central || git checkout origin/master
  $ fetch -qo- https://hg.mozilla.org/projects/nspr/raw-rev/776db96f834c | patch -Efsp1 -d nsprpub/
  $ export CC=clang60 CXX=clang++60
  $ ./mach bootstrap # select Firefox for Desktop
  $ ./mach build
  $ ./mach run

--
if ./mach bootstrap fails due to rust try updating rust package instead,
see https://clbin.com/n7tS9