--- a/build/autoconf/ffi.m4
+++ b/build/autoconf/ffi.m4
@@ -32,16 +32,22 @@ if test "$MOZ_BUILD_APP" != js -o -n "$J
       ac_configure_args="$ac_configure_args --enable-debug"
     fi
     if test "$DSO_PIC_CFLAGS"; then
       ac_configure_args="$ac_configure_args --with-pic"
     fi
     for var in AS CC CXX CPP LD AR RANLIB STRIP; do
       ac_configure_args="$ac_configure_args $var='`eval echo \\${${var}}`'"
     done
+    old_cflags="$CFLAGS"
+    # The libffi sources (especially the ARM ones) are written expecting gas
+    # syntax, and clang's integrated assembler doesn't handle all of gas syntax.
+    if test -n "$CLANG_CC" -a "$CPU_ARCH" = arm; then
+      CFLAGS="-no-integrated-as $CFLAGS"
+    fi
     if test "$CROSS_COMPILE"; then
       export CPPFLAGS CFLAGS LDFLAGS
     fi
     ac_configure_args="$ac_configure_args --build=$build --host=$target"
     if test "$_MSC_VER"; then
       # Use a wrapper script for cl and ml that looks more like gcc.
       # autotools can't quite handle an MSVC build environment yet.
       LDFLAGS=
@@ -72,13 +78,14 @@ if test "$MOZ_BUILD_APP" != js -o -n "$J
 
     # Use a separate cache file for libffi, since it does things differently
     # from our configure.
     old_config_files=$CONFIG_FILES
     unset CONFIG_FILES
     AC_OUTPUT_SUBDIRS(js/src/ctypes/libffi)
     ac_configure_args="$_SUBDIR_CONFIG_ARGS"
     CONFIG_FILES=$old_config_files
+    CFLAGS="$old_cflags"
   fi
 
 fi
 ])