diff --git a/dom/system/OSFileConstants.cpp b/dom/system/OSFileConstants.cpp
index 1e7edc29e32..ff12e438c61 100644
--- a/dom/system/OSFileConstants.cpp
+++ b/dom/system/OSFileConstants.cpp
@@ -46,6 +46,7 @@
 #include "nsIObserver.h"
 #include "nsDirectoryServiceUtils.h"
 #include "nsIXULRuntime.h"
+#include "nsIPropertyBag2.h"
 #include "nsXPCOMCIDInternal.h"
 #include "nsServiceManagerUtils.h"
 #include "nsString.h"
@@ -163,6 +164,12 @@ struct Paths {
  */
 Paths* gPaths = nullptr;
 
+/**
+ * (Unix) the umask, which goes in OS.Constants.Sys but
+ * can only be looked up (via the system-info service)
+ * on the main thread.
+ */
+uint32_t gUserUmask = 0;
 }
 
 /**
@@ -297,6 +304,19 @@ nsresult InitOSFileConstants()
 #endif // defined(XP_MACOSX)
 
   gPaths = paths.forget();
+
+  // Get the umask from the system-info service.
+  // The property will always be present, but it will be zero on
+  // non-Unix systems.
+  nsCOMPtr<nsIPropertyBag2> infoService =
+    do_GetService("@mozilla.org/system-info;1");
+  MOZ_ASSERT(infoService, "Could not access the system information service");
+  rv = infoService->GetPropertyAsUint32(NS_LITERAL_STRING("umask"),
+                                        &gUserUmask);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
+
   return NS_OK;
 }
 
@@ -853,6 +873,14 @@ bool DefineOSFileConstants(JSContext *cx, JS::Handle<JSObject*> global)
   }
 #endif
 
+  dom::ConstantSpec umask_cs[] = {
+    { "umask", UINT_TO_JSVAL(gUserUmask) },
+    PROP_END
+  };
+  if (!dom::DefineConstants(cx, objSys, umask_cs)) {
+      return false;
+  }
+
   // Build OS.Constants.Path
 
   JS::Rooted<JSObject*> objPath(cx);
diff --git a/dom/system/tests/test_constants.xul b/dom/system/tests/test_constants.xul
index e3a52ddf5df..b4b1c3dc564 100644
--- a/dom/system/tests/test_constants.xul
+++ b/dom/system/tests/test_constants.xul
@@ -64,6 +64,14 @@ function test_debugBuildMainThread(isDebugBuild) {
   is(isDebugBuild, !!OS.Constants.Sys.DEBUG, "OS.Constants.Sys.DEBUG is set properly on main thread");
 }
 
+// Test that OS.Constants.Sys.umask is set properly on main thread
+function test_umaskMainThread(umask) {
+  is(umask, OS.Constants.Sys.umask,
+     "OS.Constants.Sys.umask is set properly on main thread: " +
+     ("0000"+umask.toString(8)).slice(-4));
+}
+
+
 function test() {
   ok(true, "test_constants.xul: Starting test");
 
@@ -80,6 +88,11 @@ function test() {
                             .getService(Components.interfaces.nsIDebug2).isDebugBuild;
   test_debugBuildMainThread(isDebugBuild);
 
+  let umask = Components.classes["@mozilla.org/system-info;1"].
+    getService(Components.interfaces.nsIPropertyBag2).
+    getProperty("umask");
+  test_umaskMainThread(umask);
+
   // Test 2: Load libxul from chrome thread
   worker = new ChromeWorker("worker_constants.js");
   SimpleTest.waitForExplicitFinish();
@@ -108,8 +121,12 @@ function test() {
     }
   };
 
-  // nsIDebug2 is inaccessible from ChromeWorker. We need to pass isDebugBuild to the worker
-  worker.postMessage(isDebugBuild);
+  // pass expected values that are unavailable off-main-thread
+  // to the worker
+  worker.postMessage({
+    isDebugBuild: isDebugBuild,
+    umask: umask
+  });
   ok(true, "test_constants.xul: Test in progress");
 };
 ]]>
diff --git a/dom/system/tests/worker_constants.js b/dom/system/tests/worker_constants.js
index aafe665ea87..ace36906b90 100644
--- a/dom/system/tests/worker_constants.js
+++ b/dom/system/tests/worker_constants.js
@@ -13,11 +13,12 @@ self.onmessage = function(msg) {
   self.onmessage = function(msg) {
     log("ignored message "+JSON.stringify(msg.data));
   };
-  let isDebugBuild = msg.data;
+  let { isDebugBuild, umask } = msg.data;
   try {
     test_name();
     test_xul();
     test_debugBuildWorkerThread(isDebugBuild);
+    test_umaskWorkerThread(umask);
   } catch (x) {
     log("Catching error: " + x);
     log("Stack: " + x.stack);
@@ -51,6 +52,13 @@ function test_debugBuildWorkerThread(isDebugBuild) {
   is(isDebugBuild, !!OS.Constants.Sys.DEBUG, "OS.Constants.Sys.DEBUG is set properly on worker thread");
 }
 
+// Test that OS.Constants.Sys.umask is set properly in ChromeWorker thread
+function test_umaskWorkerThread(umask) {
+  is(umask, OS.Constants.Sys.umask,
+     "OS.Constants.Sys.umask is set properly on worker thread: " +
+     ("0000"+umask.toString(8)).slice(-4));
+}
+
 // Test that OS.Constants.Path.libxul lets us open libxul
 function test_xul() {
   let lib;