diff -Nru mozilla/config/external/moz.build mozilla-OK/config/external/moz.build
--- mozilla/config/external/moz.build	2020-10-20 22:17:57.000000000 +0300
+++ mozilla-OK/config/external/moz.build	2020-11-12 19:23:45.888994819 +0300
@@ -36,8 +36,9 @@
     external_dirs += ['media/libvpx']
 
 if CONFIG['MOZ_AV1']:
-    external_dirs += ['media/libaom']
-    external_dirs += ['media/libdav1d']
+    if not CONFIG['MOZ_SYSTEM_AV1']:
+        external_dirs += ['media/libaom']
+        external_dirs += ['media/libdav1d']
 
 if not CONFIG['MOZ_SYSTEM_PNG']:
     external_dirs += ['media/libpng']
diff -Nru mozilla/config/system-headers.mozbuild mozilla-OK/config/system-headers.mozbuild
--- mozilla/config/system-headers.mozbuild	2020-10-20 22:17:57.000000000 +0300
+++ mozilla-OK/config/system-headers.mozbuild	2020-11-12 19:23:45.888994819 +0300
@@ -1321,6 +1321,14 @@
         'pixman.h',
     ]
 
+if CONFIG['MOZ_SYSTEM_AV1']:
+    system_headers += [
+        'aom/aom_decoder.h',
+        'aom/aomdx.h',
+        'aom/aom_image.h',
+        'dav1d/dav1d.h',
+    ]
+
 if CONFIG['MOZ_SYSTEM_LIBVPX']:
     system_headers += [
         'vpx_mem/vpx_mem.h',
diff -Nru mozilla/dom/media/platforms/moz.build mozilla-OK/dom/media/platforms/moz.build
--- mozilla/dom/media/platforms/moz.build	2020-10-20 22:17:57.000000000 +0300
+++ mozilla-OK/dom/media/platforms/moz.build	2020-11-12 19:23:45.889994811 +0300
@@ -74,6 +74,11 @@
         'agnostic/AOMDecoder.cpp',
         'agnostic/DAV1DDecoder.cpp',
     ]
+    if CONFIG['MOZ_SYSTEM_AV1']:
+        CXXFLAGS += CONFIG['MOZ_SYSTEM_LIBAOM_CFLAGS']
+        OS_LIBS += CONFIG['MOZ_SYSTEM_LIBAOM_LIBS']
+        CXXFLAGS += CONFIG['MOZ_SYSTEM_LIBDAV1D_CFLAGS']
+        OS_LIBS += CONFIG['MOZ_SYSTEM_LIBDAV1D_LIBS']
 
 if CONFIG['MOZ_APPLEMEDIA']:
   EXPORTS += [
diff -Nru mozilla/toolkit/moz.configure mozilla-OK/toolkit/moz.configure
--- mozilla/toolkit/moz.configure	2020-10-21 23:09:59.000000000 +0300
+++ mozilla-OK/toolkit/moz.configure	2020-11-12 19:23:45.889994811 +0300
@@ -370,7 +370,23 @@
     if value:
         return True
 
-@depends(target, nasm_version, when=av1 & compile_environment)
+option('--with-system-av1',
+       help="Use system av1 (located with pkgconfig)")
+
+system_libaom_info = pkg_check_modules('MOZ_SYSTEM_LIBAOM', 'aom >= 1.0.0',
+                                       when='--with-system-av1')
+
+system_libdav1d_info = pkg_check_modules('MOZ_SYSTEM_LIBDAV1D', 'dav1d >= 0.1.1',
+                                         when='--with-system-av1')
+
+@depends(system_libaom_info, system_libdav1d_info)
+def system_av1(system_libaom_info, system_libdav1d_info):
+    has_av1_libs = False
+    if system_libaom_info and system_libdav1d_info:
+        has_av1_libs = True
+    return has_av1_libs
+
+@depends(target, nasm_version, when=av1 & depends(system_av1)(lambda v: not v) & compile_environment)
 def dav1d_asm(target, nasm_version):
     if target.os != 'Android':
         if target.cpu == 'aarch64':
@@ -386,6 +402,7 @@
 set_define('MOZ_DAV1D_ASM', dav1d_asm)
 set_config('MOZ_AV1', av1)
 set_define('MOZ_AV1', av1)
+set_config('MOZ_SYSTEM_AV1', depends_if(system_av1)(lambda _: True))
 
 # Built-in fragmented MP4 support.
 # ==============================================================