--- a/media/libvpx/moz.build
+++ b/media/libvpx/moz.build
@@ -40,16 +40,23 @@ if CONFIG['VPX_X86_ASM']:
     if '64' in CONFIG['OS_TEST']:
         SOURCES += files['X86-64_ASM']
 
     # AVX2 only supported on
     # Darwin toolchain right now
     if CONFIG['OS_TARGET'] == 'Darwin':
         SOURCES += files['AVX2']
 
+    # Expected support is hard-coded in the various vpx_config files but
+    # we need to propagate the config checks here to get the right flags.
+    if CONFIG['HAVE_TOOLCHAIN_SUPPORT_MSSSE3']:
+        CFLAGS += ['-mssse3']
+    if CONFIG['HAVE_TOOLCHAIN_SUPPORT_MSSE4_1']:
+        CFLAGS += ['-msse4.1']
+
     #postproc is only enabled on x86 with asm
     SOURCES += files['VP8_POSTPROC']
 
 arm_asm_files = []
 if CONFIG['VPX_ARM_ASM']:
     arm_asm_files += files['ARM_ASM']
 
     if CONFIG['VPX_AS_CONVERSION']: