I am bisecting this should have a regression check-in identified soon.

This is very odd. There could be multiple issues here. Not only does the build fail, but it also loops forever repeating the error.

Oh. and I have no idea if this is specifically express related as I have no Pro version to test with.

HG bisect says: The first bad revision is: ←[0;33mchangeset: 160361:67fa1478308e←[0m user: Gregory Szorc <gps@mozilla.com> date: Thu Dec 12 16:26:38 2013 +0900 summary: Bug 928195 - Part 4: Rewrite WebIDL build system integration; r=bz, r=glandium

Here is my .mozconfig . $topsrcdir/browser/config/mozconfig BUILD_OFFICIAL=1 export BUILD_OFFICIAL WIN32_REDIST_DIR=~/mozilla/redist/x86 export WIN32_REDIST_DIR #WIN32_CRT_SRC_DIR=~/mozilla/crtsrc mk_add_options BUILD_OFFICIAL=1 mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/fx-obj mk_add_options RUN_AUTOCONF_LOCALLY=1 mk_add_options AUTOCONF=/usr/local/bin/autoconf-2.13 ac_add_options --enable-optimize=-O2 ac_add_options --disable-debug ac_add_options --disable-tests ac_add_options --disable-crashreporter ac_add_options --enable-metro ac_add_options --enable-jemalloc ac_add_options --without-x mk_add_options MOZ_MAKE_FLAGS="-j3"