--- a/toolkit/xre/nsXREDirProvider.cpp
+++ b/toolkit/xre/nsXREDirProvider.cpp
@@ -473,16 +473,19 @@ nsXREDirProvider::GetFile(const char* aP
 #endif
   else if (!strcmp(aProperty, XRE_USER_SYS_EXTENSION_DIR)) {
 #ifdef ENABLE_SYSTEM_EXTENSION_DIRS
     return GetSysUserExtensionsDirectory(aFile);
 #else
     return NS_ERROR_FAILURE;
 #endif
   }
+  else if (!strcmp(aProperty, XRE_USER_SYS_EXTENSION_DEV_DIR)) {
+    return GetSysUserExtensionsDevDirectory(aFile);
+  }
   else if (!strcmp(aProperty, XRE_APP_DISTRIBUTION_DIR)) {
     bool persistent = false;
     rv = GetFile(NS_GRE_DIR, &persistent, getter_AddRefs(file));
     if (NS_SUCCEEDED(rv))
       rv = file->AppendNative(NS_LITERAL_CSTRING("distribution"));
   }
   else if (!strcmp(aProperty, XRE_APP_FEATURES_DIR)) {
     rv = GetAppDir()->Clone(getter_AddRefs(file));
@@ -1517,16 +1520,33 @@ nsXREDirProvider::GetSysUserExtensionsDi
 
   rv = EnsureDirectoryExists(localDir);
   NS_ENSURE_SUCCESS(rv, rv);
 
   localDir.forget(aFile);
   return NS_OK;
 }
 
+nsresult
+nsXREDirProvider::GetSysUserExtensionsDevDirectory(nsIFile** aFile)
+{
+  nsCOMPtr<nsIFile> localDir;
+  nsresult rv = GetUserDataDirectoryHome(getter_AddRefs(localDir), false);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = AppendSysUserExtensionsDevPath(localDir);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = EnsureDirectoryExists(localDir);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  localDir.forget(aFile);
+  return NS_OK;
+}
+
 #if defined(XP_UNIX) || defined(XP_MACOSX)
 nsresult
 nsXREDirProvider::GetSystemExtensionsDirectory(nsIFile** aFile)
 {
   nsresult rv;
   nsCOMPtr<nsIFile> localDir;
 
   rv = GetSystemParentDirectory(getter_AddRefs(localDir));
@@ -1624,16 +1644,49 @@ nsXREDirProvider::AppendSysUserExtension
   NS_ENSURE_SUCCESS(rv, rv);
 
 #else
 #error "Don't know how to get XRE user extension path on your platform"
 #endif
   return NS_OK;
 }
 
+nsresult
+nsXREDirProvider::AppendSysUserExtensionsDevPath(nsIFile* aFile)
+{
+  MOZ_ASSERT(aFile);
+
+  nsresult rv;
+
+#if defined (XP_MACOSX) || defined(XP_WIN)
+
+  static const char* const sXR = "Mozilla";
+  rv = aFile->AppendNative(nsDependentCString(sXR));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  static const char* const sExtensions = "SystemExtensionsDev";
+  rv = aFile->AppendNative(nsDependentCString(sExtensions));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+#elif defined(XP_UNIX)
+
+  static const char* const sXR = ".mozilla";
+  rv = aFile->AppendNative(nsDependentCString(sXR));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  static const char* const sExtensions = "systemextensionsdev";
+  rv = aFile->AppendNative(nsDependentCString(sExtensions));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+#else
+#error "Don't know how to get XRE system extension dev path on your platform"
+#endif
+  return NS_OK;
+}
+
 
 nsresult
 nsXREDirProvider::AppendProfilePath(nsIFile* aFile,
                                     const nsACString* aProfileName,
                                     const nsACString* aAppName,
                                     const nsACString* aVendorName,
                                     bool aLocal)
 {

--- a/toolkit/xre/nsXREDirProvider.h
+++ b/toolkit/xre/nsXREDirProvider.h
@@ -99,30 +99,32 @@ public:
    * when XPCOM is initialized! aResult is a clone, it may be modified.
    */
   nsresult GetProfileDir(nsIFile* *aResult);
 
 protected:
   nsresult GetFilesInternal(const char* aProperty, nsISimpleEnumerator** aResult);
   static nsresult GetUserDataDirectoryHome(nsIFile* *aFile, bool aLocal);
   static nsresult GetSysUserExtensionsDirectory(nsIFile* *aFile);
+  static nsresult GetSysUserExtensionsDevDirectory(nsIFile* *aFile);
 #if defined(XP_UNIX) || defined(XP_MACOSX)
   static nsresult GetSystemExtensionsDirectory(nsIFile** aFile);
 #endif
   static nsresult EnsureDirectoryExists(nsIFile* aDirectory);
 
   // Determine the profile path within the UAppData directory. This is different
   // on every major platform.
   static nsresult AppendProfilePath(nsIFile* aFile,
                                     const nsACString* aProfileName,
                                     const nsACString* aAppName,
                                     const nsACString* aVendorName,
                                     bool aLocal);
 
   static nsresult AppendSysUserExtensionPath(nsIFile* aFile);
+  static nsresult AppendSysUserExtensionsDevPath(nsIFile* aFile);
 
   // Internal helper that splits a path into components using the '/' and '\\'
   // delimiters.
   static inline nsresult AppendProfileString(nsIFile* aFile, const char* aPath);
 
 #if (defined(XP_WIN) || defined(XP_MACOSX)) && defined(MOZ_CONTENT_SANDBOX)
   // Load the temp directory for sandboxed content processes
   nsresult LoadContentProcessTempDir();

--- a/xpcom/build/nsXULAppAPI.h
+++ b/xpcom/build/nsXULAppAPI.h
@@ -124,16 +124,22 @@
 
 /**
  * A directory service key which specifies the user system extension
  * parent directory.
  */
 #define XRE_USER_SYS_EXTENSION_DIR "XREUSysExt"
 
 /**
+ * A directory service key which specifies a directory where temporary
+ * system extensions can be loaded from during development.
+ */
+#define XRE_USER_SYS_EXTENSION_DEV_DIR "XRESysExtDev"
+
+/**
  * A directory service key which specifies the distribution specific files for
  * the application.
  */
 #define XRE_APP_DISTRIBUTION_DIR "XREAppDist"
 
 /**
  * A directory service key which specifies the location for system add-ons.
  */