--- a/security/sandbox/win/src/sandboxbroker/sandboxBroker.cpp
+++ b/security/sandbox/win/src/sandboxbroker/sandboxBroker.cpp
@@ -45,16 +45,17 @@ sandbox::BrokerServices *SandboxBroker::
 bool SandboxBroker::sRunningFromNetworkDrive = false;
 
 // Cached special directories used for adding policy rules.
 static UniquePtr<nsString> sBinDir;
 static UniquePtr<nsString> sProfileDir;
 static UniquePtr<nsString> sContentTempDir;
 static UniquePtr<nsString> sRoamingAppDataDir;
 static UniquePtr<nsString> sLocalAppDataDir;
+static UniquePtr<nsString> sUserExtensionsDevDir;
 #ifdef ENABLE_SYSTEM_EXTENSION_DIRS
 static UniquePtr<nsString> sUserExtensionsDir;
 #endif
 
 static LazyLogModule sSandboxBrokerLog("SandboxBroker");
 
 #define LOG_E(...) MOZ_LOG(sSandboxBrokerLog, LogLevel::Error, (__VA_ARGS__))
 #define LOG_W(...) MOZ_LOG(sSandboxBrokerLog, LogLevel::Warning, (__VA_ARGS__))
@@ -125,16 +126,17 @@ SandboxBroker::CacheRulesDirectories()
     return;
   }
 
   CacheDirAndAutoClear(dirSvc, NS_GRE_DIR, &sBinDir);
   CacheDirAndAutoClear(dirSvc, NS_APP_USER_PROFILE_50_DIR, &sProfileDir);
   CacheDirAndAutoClear(dirSvc, NS_APP_CONTENT_PROCESS_TEMP_DIR, &sContentTempDir);
   CacheDirAndAutoClear(dirSvc, NS_WIN_APPDATA_DIR, &sRoamingAppDataDir);
   CacheDirAndAutoClear(dirSvc, NS_WIN_LOCAL_APPDATA_DIR, &sLocalAppDataDir);
+  CacheDirAndAutoClear(dirSvc, XRE_USER_SYS_EXTENSION_DEV_DIR, &sUserExtensionsDevDir);
 #ifdef ENABLE_SYSTEM_EXTENSION_DIRS
   CacheDirAndAutoClear(dirSvc, XRE_USER_SYS_EXTENSION_DIR, &sUserExtensionsDir);
 #endif
 }
 
 SandboxBroker::SandboxBroker()
 {
   if (sBrokerService) {
@@ -494,16 +496,20 @@ SandboxBroker::SetSecurityLevelForConten
     // Add rule to allow read access to the chrome directory within profile.
     AddCachedDirRule(mPolicy, sandbox::TargetPolicy::FILES_ALLOW_READONLY,
                      sProfileDir, NS_LITERAL_STRING("\\chrome\\*"));
 
     // Add rule to allow read access to the extensions directory within profile.
     AddCachedDirRule(mPolicy, sandbox::TargetPolicy::FILES_ALLOW_READONLY,
                      sProfileDir, NS_LITERAL_STRING("\\extensions\\*"));
 
+    // Read access to a directory for system extension dev (see bug 1393805)
+    AddCachedDirRule(mPolicy, sandbox::TargetPolicy::FILES_ALLOW_READONLY,
+                     sUserExtensionsDevDir, NS_LITERAL_STRING("\\*"));
+
 #ifdef ENABLE_SYSTEM_EXTENSION_DIRS
     // Add rule to allow read access to the per-user extensions directory.
     AddCachedDirRule(mPolicy, sandbox::TargetPolicy::FILES_ALLOW_READONLY,
                      sUserExtensionsDir, NS_LITERAL_STRING("\\*"));
 #endif
   }
 
   // Add the policy for the client side of a pipe. It is just a file