On a freshly-clobbered objdir, I'm getting the following when I try to build: 28:47.83 c:/mozbuild/src/mozilla-central/layout/build/nsLayoutModule.cpp(98) : fatal error C1083: Cannot open include file: 'mozilla/dom/FakeSpeechRecognitionService.h': No such file or directory Using MSVC 2012 on Windows 7 x64. My .mozconfig if relevant: # # Basic Build Options # mk_add_options MOZ_CO_PROJECT=browser mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/objdir-fx ac_add_options --enable-application=browser # # Use Official Branding # ac_add_options --enable-official-branding mk_add_options MOZILLA_OFFICIAL=1 mk_add_options BUILD_OFFICIAL=1 # # Build Optimization Settings # ac_add_options --enable-optimize="-O2 -GLS -arch:SSE2" mk_add_options MOZ_MAKE_FLAGS="-s -j 12" # # Enable or Disable Debug # ac_add_options --enable-debug-symbols ac_add_options --disable-debug # # Enable or Disable Tests # ac_add_options --disable-mochitest ac_add_options --disable-tests # # Other Build Options # ac_add_options --disable-accessibility ac_add_options --disable-crashreporter ac_add_options --disable-parental-controls ac_add_options --disable-safe-browsing ac_add_options --disable-updater

Looks like a dupe of bug 855728, which has a patch.