--- a/security/sandbox/linux/broker/SandboxBrokerPolicyFactory.cpp
+++ b/security/sandbox/linux/broker/SandboxBrokerPolicyFactory.cpp
@@ -254,28 +254,40 @@ SandboxBrokerPolicyFactory::SandboxBroke
         }
       }
     }
   }
 
   // Firefox binary dir.
   // Note that unlike the previous cases, we use NS_GetSpecialDirectory
   // instead of GetSpecialSystemDirectory. The former requires a working XPCOM
-  // system, which may not be the case for some tests. For quering for the
+  // system, which may not be the case for some tests. For querying for the
   // location of XPCOM things, we can use it anyway.
   nsCOMPtr<nsIFile> ffDir;
   rv = NS_GetSpecialDirectory(NS_GRE_DIR, getter_AddRefs(ffDir));
   if (NS_SUCCEEDED(rv)) {
     nsAutoCString tmpPath;
     rv = ffDir->GetNativePath(tmpPath);
     if (NS_SUCCEEDED(rv)) {
       policy->AddDir(rdonly, tmpPath.get());
     }
   }
 
+  // ~/.mozilla/systemextensionsdev (bug 1393805)
+  nsCOMPtr<nsIFile> sysExtDevDir;
+  rv = NS_GetSpecialDirectory(XRE_USER_SYS_EXTENSION_DEV_DIR,
+                              getter_AddRefs(sysExtDevDir));
+  if (NS_SUCCEEDED(rv)) {
+    nsAutoCString tmpPath;
+    rv = sysExtDevDir->GetNativePath(tmpPath);
+    if (NS_SUCCEEDED(rv)) {
+      policy->AddDir(rdonly, tmpPath.get());
+    }
+  }
+
   if (mozilla::IsDevelopmentBuild()) {
     // If this is a developer build the resources are symlinks to outside the binary dir.
     // Therefore in non-release builds we allow reads from the whole repository.
     // MOZ_DEVELOPER_REPO_DIR is set by mach run.
     const char *developer_repo_dir = PR_GetEnv("MOZ_DEVELOPER_REPO_DIR");
     if (developer_repo_dir) {
       policy->AddDir(rdonly, developer_repo_dir);
     }