gistfile1.diff

diff --git a/config/external/moz.build b/config/external/moz.build
--- a/config/external/moz.build
+++ b/config/external/moz.build
@@ -8,12 +8,16 @@ external_dirs = []
 
 DIRS += [
     'lgpllibs',
-    'prio',
     'sqlite',
 ]
 if not CONFIG['MOZ_SYSTEM_JPEG']:
     external_dirs += ['media/libjpeg']
 
+if CONFIG['MOZ_LIBPRIO']:
+    DIRS += [
+        'prio',
+    ]
+
 # There's no "native" brotli or woff2 yet, but probably in the future...
 external_dirs += ['modules/brotli']
 external_dirs += ['modules/woff2']
diff --git a/config/external/prio/moz.build b/config/external/prio/moz.build
--- a/config/external/prio/moz.build
+++ b/config/external/prio/moz.build
@@ -4,5 +4,5 @@
 # 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/.
 
-DIRS += ['../../../third_party/prio']
+DIRS += ['/third_party/prio']
 
diff --git a/dom/chrome-webidl/moz.build b/dom/chrome-webidl/moz.build
--- a/dom/chrome-webidl/moz.build
+++ b/dom/chrome-webidl/moz.build
@@ -44,7 +44,6 @@ WEBIDL_FILES = [
     'MozStorageStatementParams.webidl',
     'MozStorageStatementRow.webidl',
     'PrecompiledScript.webidl',
-    'PrioEncoder.webidl',
     'PromiseDebugging.webidl',
     'StructuredCloneHolder.webidl',
     'WebExtensionContentScript.webidl',
@@ -58,3 +57,9 @@ if CONFIG['MOZ_PLACES']:
         'PlacesEvent.webidl',
         'PlacesObservers.webidl',
     ]
+
+if CONFIG['MOZ_LIBPRIO']:
+    WEBIDL_FILES += [
+        'PrioEncoder.webidl',
+    ]
+
diff --git a/dom/moz.build b/dom/moz.build
--- a/dom/moz.build
+++ b/dom/moz.build
@@ -63,7 +63,6 @@ DIRS += [
     'notification',
     'offline',
     'power',
-    'prio',
     'push',
     'quota',
     'security',
@@ -105,6 +104,9 @@ DIRS += [
     'simpledb',
 ]
 
+if CONFIG['MOZ_LIBPRIO']:
+    DIRS += 'prio'
+
 if CONFIG['OS_ARCH'] == 'WINNT':
     DIRS += ['plugins/ipc/hangui']
 
diff --git a/toolkit/moz.configure b/toolkit/moz.configure
--- a/toolkit/moz.configure
+++ b/toolkit/moz.configure
@@ -1064,3 +1064,14 @@ def launcher(value, target):
 
 set_config('MOZ_LAUNCHER_PROCESS', launcher)
 set_define('MOZ_LAUNCHER_PROCESS', launcher)
+
+# Prio
+# ==============================================================
+@depends(c_compiler)
+def libprio(info):
+    if info.type in ('msvc'):
+        return False
+    return True
+
+set_define('MOZ_LIBPRIO', libprio)
+