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diff -up firefox-91.0/extensions/pref/autoconfig/src/nsReadConfig.cpp.1170092 firefox-91.0/extensions/pref/autoconfig/src/nsReadConfig.cpp
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--- firefox-91.0/extensions/pref/autoconfig/src/nsReadConfig.cpp.1170092	2021-07-27 23:29:57.000000000 +0200
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+++ firefox-91.0/extensions/pref/autoconfig/src/nsReadConfig.cpp	2021-07-29 16:20:44.164513998 +0200
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@@ -249,8 +249,20 @@ nsresult nsReadConfig::openAndEvaluateJS
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     if (NS_FAILED(rv)) return rv;
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     rv = NS_NewLocalFileInputStream(getter_AddRefs(inStr), jsFile);
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-    if (NS_FAILED(rv)) return rv;
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+    if (NS_FAILED(rv)) {
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+      // Look for cfg file in /etc/<application>/pref
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+      rv = NS_GetSpecialDirectory(NS_APP_PREFS_SYSTEM_CONFIG_DIR,
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+                                  getter_AddRefs(jsFile));
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+      NS_ENSURE_SUCCESS(rv, rv);
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+
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+      rv = jsFile->AppendNative(nsLiteralCString("pref"));
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+      NS_ENSURE_SUCCESS(rv, rv);
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+      rv = jsFile->AppendNative(nsDependentCString(aFileName));
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+      NS_ENSURE_SUCCESS(rv, rv);
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+      rv = NS_NewLocalFileInputStream(getter_AddRefs(inStr), jsFile);
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+      NS_ENSURE_SUCCESS(rv, rv);
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+    }
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   } else {
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     nsAutoCString location("resource://gre/defaults/autoconfig/");
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     location += aFileName;
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diff -up firefox-91.0/modules/libpref/Preferences.cpp.1170092 firefox-91.0/modules/libpref/Preferences.cpp
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--- firefox-91.0/modules/libpref/Preferences.cpp.1170092	2021-07-27 23:30:34.000000000 +0200
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+++ firefox-91.0/modules/libpref/Preferences.cpp	2021-07-29 16:20:44.166514005 +0200
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@@ -4553,6 +4553,8 @@ nsresult Preferences::InitInitialObjects
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   //
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   // Thus, in the omni.jar case, we always load app-specific default
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   // preferences from omni.jar, whether or not `$app == $gre`.
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+  // At very end load configuration from system config location:
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+  // - /etc/firefox/pref/*.js
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   nsresult rv = NS_ERROR_FAILURE;
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   UniquePtr<nsZipFind> find;
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diff -up firefox-91.0/toolkit/xre/nsXREDirProvider.cpp.1170092 firefox-91.0/toolkit/xre/nsXREDirProvider.cpp
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--- firefox-91.0/toolkit/xre/nsXREDirProvider.cpp.1170092	2021-07-27 23:30:56.000000000 +0200
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+++ firefox-91.0/toolkit/xre/nsXREDirProvider.cpp	2021-07-29 16:26:20.071628932 +0200
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@@ -71,6 +71,7 @@
fb5326
 #endif
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 #ifdef XP_UNIX
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 #  include <ctype.h>
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+#  include "nsIXULAppInfo.h"
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 #endif
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 #ifdef XP_IOS
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 #  include "UIKitDirProvider.h"
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@@ -550,6 +551,21 @@ nsXREDirProvider::GetFile(const char* aP
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       }
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     }
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   }
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+
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+#if defined(XP_UNIX)
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+  if (!strcmp(aProperty, NS_APP_PREFS_SYSTEM_CONFIG_DIR)) {
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+    nsCString sysConfigDir = nsLiteralCString("/etc/");
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+    nsCOMPtr<nsIXULAppInfo> appInfo = do_GetService("@mozilla.org/xre/app-info;1");
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+    if (!appInfo)
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+      return NS_ERROR_NOT_AVAILABLE;
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+    nsCString appName;
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+    appInfo->GetName(appName);
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+    ToLowerCase(appName);
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+    sysConfigDir.Append(appName);
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+    return NS_NewNativeLocalFile(sysConfigDir, false, aFile);
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+  }
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+#endif
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+
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   if (NS_FAILED(rv) || !file) return NS_ERROR_FAILURE;
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   if (ensureFilePermissions) {
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@@ -827,6 +843,16 @@ nsresult nsXREDirProvider::GetFilesInter
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       LoadDirIntoArray(mXULAppDir, kAppendBackgroundTasksPrefDir, directories);
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     }
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 #endif
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+    // Add /etc/<application>/pref/ directory if it exists
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+    nsCOMPtr<nsIFile> systemPrefDir;
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+    rv = NS_GetSpecialDirectory(NS_APP_PREFS_SYSTEM_CONFIG_DIR,
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+                                getter_AddRefs(systemPrefDir));
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+    if (NS_SUCCEEDED(rv)) {
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+      rv = systemPrefDir->AppendNative(nsLiteralCString("pref"));
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+      if (NS_SUCCEEDED(rv)) {
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+        directories.AppendObject(systemPrefDir);
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+      }
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+    }
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     rv = NS_NewArrayEnumerator(aResult, directories, NS_GET_IID(nsIFile));
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   } else if (!strcmp(aProperty, NS_APP_CHROME_DIR_LIST)) {
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diff -up firefox-91.0/xpcom/io/nsAppDirectoryServiceDefs.h.1170092 firefox-91.0/xpcom/io/nsAppDirectoryServiceDefs.h
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--- firefox-91.0/xpcom/io/nsAppDirectoryServiceDefs.h.1170092	2021-07-27 23:31:38.000000000 +0200
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+++ firefox-91.0/xpcom/io/nsAppDirectoryServiceDefs.h	2021-07-29 16:20:44.168514012 +0200
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@@ -59,6 +59,7 @@
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 #define NS_APP_PREFS_DEFAULTS_DIR_LIST "PrefDL"
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 #define NS_APP_PREFS_OVERRIDE_DIR \
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   "PrefDOverride"  // Directory for per-profile defaults
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+#define NS_APP_PREFS_SYSTEM_CONFIG_DIR          "PrefSysConf"   // Directory with system-wide configuration
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 #define NS_APP_USER_PROFILE_50_DIR "ProfD"
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 #define NS_APP_USER_PROFILE_LOCAL_50_DIR "ProfLD"