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diff -up firefox-87.0/extensions/pref/autoconfig/src/nsReadConfig.cpp.1170092 firefox-87.0/extensions/pref/autoconfig/src/nsReadConfig.cpp
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--- firefox-87.0/extensions/pref/autoconfig/src/nsReadConfig.cpp.1170092	2021-03-18 14:48:36.000000000 +0100
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+++ firefox-87.0/extensions/pref/autoconfig/src/nsReadConfig.cpp	2021-03-22 19:20:02.429310184 +0100
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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-87.0/modules/libpref/Preferences.cpp.1170092 firefox-87.0/modules/libpref/Preferences.cpp
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--- firefox-87.0/modules/libpref/Preferences.cpp.1170092	2021-03-18 14:48:54.000000000 +0100
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+++ firefox-87.0/modules/libpref/Preferences.cpp	2021-03-22 19:20:02.429310184 +0100
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@@ -4499,6 +4499,9 @@ 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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+  //
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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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025a5f
   nsresult rv = NS_ERROR_FAILURE;
f1b26c
   UniquePtr<nsZipFind> find;
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diff -up firefox-87.0/toolkit/xre/nsXREDirProvider.cpp.1170092 firefox-87.0/toolkit/xre/nsXREDirProvider.cpp
517bf9
--- firefox-87.0/toolkit/xre/nsXREDirProvider.cpp.1170092	2021-03-18 14:52:00.000000000 +0100
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+++ firefox-87.0/toolkit/xre/nsXREDirProvider.cpp	2021-03-22 19:37:56.574480347 +0100
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@@ -65,6 +65,7 @@
fb5326
 #endif
fb5326
 #ifdef XP_UNIX
fb5326
 #  include <ctype.h>
fb5326
+#  include "nsIXULAppInfo.h"
fb5326
 #endif
fb5326
 #ifdef XP_IOS
fb5326
 #  include "UIKitDirProvider.h"
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@@ -552,6 +553,21 @@ 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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@@ -874,6 +890,16 @@ nsresult nsXREDirProvider::GetFilesInter
8a9c29
     }
8a9c29
 #endif
517bf9
 
969f7f
+    // 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"));
517bf9
+      if (NS_SUCCEEDED(rv))
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+        directories.AppendObject(systemPrefDir);
f1b26c
+      }
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+
025a5f
     rv = NS_NewArrayEnumerator(aResult, directories, NS_GET_IID(nsIFile));
969f7f
   } else if (!strcmp(aProperty, NS_APP_CHROME_DIR_LIST)) {
517bf9
     // NS_APP_CHROME_DIR_LIST is only used to get default (native) icons
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diff -up firefox-87.0/xpcom/io/nsAppDirectoryServiceDefs.h.1170092 firefox-87.0/xpcom/io/nsAppDirectoryServiceDefs.h
517bf9
--- firefox-87.0/xpcom/io/nsAppDirectoryServiceDefs.h.1170092	2021-03-18 14:51:58.000000000 +0100
517bf9
+++ firefox-87.0/xpcom/io/nsAppDirectoryServiceDefs.h	2021-03-22 19:20:02.430310213 +0100
8a9c29
@@ -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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969f7f
 #define NS_APP_USER_PROFILE_50_DIR "ProfD"
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 #define NS_APP_USER_PROFILE_LOCAL_50_DIR "ProfLD"