diff -up firefox-122.0/config/external/moz.build.system-av1 firefox-122.0/config/external/moz.build
--- firefox-122.0/config/external/moz.build.system-av1	2024-01-18 21:41:20.000000000 +0100
+++ firefox-122.0/config/external/moz.build	2024-01-19 18:27:03.512034790 +0100
@@ -40,8 +40,9 @@ if not CONFIG["MOZ_SYSTEM_LIBVPX"]:
     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 -up firefox-122.0/config/system-headers.mozbuild.system-av1 firefox-122.0/config/system-headers.mozbuild
--- firefox-122.0/config/system-headers.mozbuild.system-av1	2024-01-19 18:27:03.513034826 +0100
+++ firefox-122.0/config/system-headers.mozbuild	2024-01-19 19:02:54.515493457 +0100
@@ -1304,6 +1304,14 @@ if CONFIG["MOZ_ENABLE_LIBPROXY"]:
         "proxy.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_ICU"]:
     system_headers += [
         "unicode/calendar.h",
diff -up firefox-122.0/dom/media/platforms/moz.build.system-av1 firefox-122.0/dom/media/platforms/moz.build
--- firefox-122.0/dom/media/platforms/moz.build.system-av1	2024-01-18 21:41:21.000000000 +0100
+++ firefox-122.0/dom/media/platforms/moz.build	2024-01-19 18:27:03.513034826 +0100
@@ -81,6 +81,11 @@ if CONFIG["MOZ_AV1"]:
         "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_OMX"]:
     EXPORTS += [
diff -up firefox-122.0/toolkit/moz.configure.system-av1 firefox-122.0/toolkit/moz.configure
--- firefox-122.0/toolkit/moz.configure.system-av1	2024-01-19 18:27:03.495034173 +0100
+++ firefox-122.0/toolkit/moz.configure	2024-01-19 18:27:03.514034863 +0100
@@ -743,14 +743,29 @@ def av1(value):
     if value:
         return True
 
+option("--with-system-av1", help="Use system av1 (located with pkg-config)")
 
-@depends(target, when=av1 & compile_environment)
+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, when=av1 & depends(system_av1)(lambda v: not v) & compile_environment)
 def dav1d_asm(target):
     if target.cpu in ("aarch64", "x86", "x86_64"):
         return True
 
 
-@depends(target, when=av1 & compile_environment)
+@depends(target, when=av1 & depends(system_av1)(lambda v: not v) & compile_environment)
 def dav1d_nasm(target):
     if target.cpu in ("x86", "x86_64"):
         return namespace(version="2.14", what="AV1")
@@ -760,6 +775,7 @@ set_config("MOZ_DAV1D_ASM", dav1d_asm)
 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))
 
 # JXL Image Codec Support
 # ==============================================================