--- a/media/ffvpx/config.h
+++ b/media/ffvpx/config.h
@@ -2,21 +2,35 @@
 /* vim:set ts=2 sw=2 sts=2 et cindent: */
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #ifndef MOZ_FFVPX_CONFIG_H
 #define MOZ_FFVPX_CONFIG_H
 #if defined(XP_WIN)
+// Avoid conflicts with mozilla-config.h
+#if !defined(_MSC_VER)
+#undef HAVE_DIRENT_H
+#undef HAVE_UNISTD_H
+#endif
 #if defined(HAVE_64BIT_BUILD)
 #include "config_win64.h"
 #else
 #include "config_win32.h"
 #endif
+// Adjust configure defines for GCC
+#if !defined(_MSC_VER)
+#if !defined(HAVE_64BIT_BUILD)
+#undef HAVE_MM_EMPTY
+#define HAVE_MM_EMPTY 0
+#endif
+#undef HAVE_LIBC_MSVCRT
+#define HAVE_LIBC_MSVCRT 0
+#endif
 #elif defined(XP_DARWIN)
 #if defined(HAVE_64BIT_BUILD)
 #include "config_darwin64.h"
 #else
 #include "config_darwin32.h"
 #endif
 #elif defined(XP_UNIX)
 #if defined(HAVE_64BIT_BUILD)

--- a/media/ffvpx/libavutil/moz.build
+++ b/media/ffvpx/libavutil/moz.build
@@ -46,17 +46,17 @@ SOURCES += [
     'reverse.c',
     'samplefmt.c',
     'threadmessage.c',
     'time.c',
     'timecode.c',
     'utils.c',
 ]
 
-if CONFIG['_MSC_VER']:
+if CONFIG['OS_ARCH'] == 'WINNT':
     SOURCES += [
         '../compat/strtod.c'
     ]
     if CONFIG['_MSC_VER'] < '1900':
         SOURCES += [
             '../compat/msvcrt/snprintf.c'
         ]