diff --git a/browser/base/content/browser.js b/browser/base/content/browser.js
index 39d49200d2d4..8d3b177a4468 100644
--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -6873,7 +6873,14 @@ let gPrivateBrowsingUI = {
     var brandBundle = bundleService.createBundle("chrome://branding/locale/brand.properties");
 
     var appName = brandBundle.GetStringFromName("brandShortName");
+# On Mac, use the header as the title.
+#ifdef XP_MACOSX
+    var dialogTitle = pbBundle.GetStringFromName("privateBrowsingMessageHeader");
+    var header = "";
+#else
     var dialogTitle = pbBundle.GetStringFromName("privateBrowsingDialogTitle");
+    var header = pbBundle.GetStringFromName("privateBrowsingMessageHeader") + "\n\n";
+#endif
     var message = pbBundle.formatStringFromName("privateBrowsingMessage", [appName], 1);
 
     var promptService = Cc["@mozilla.org/embedcomp/prompt-service;1"].
@@ -6889,7 +6896,7 @@ let gPrivateBrowsingUI = {
     var neverAskText = pbBundle.GetStringFromName("privateBrowsingNeverAsk");
 
     var result;
-    var choice = promptService.confirmEx(null, dialogTitle, message,
+    var choice = promptService.confirmEx(null, dialogTitle, header + message,
                                flags, button0Title, button1Title, null,
                                neverAskText, neverAsk);
 
diff --git a/browser/locales/en-US/chrome/browser/browser.properties b/browser/locales/en-US/chrome/browser/browser.properties
index e649ca21b661..9c6b9bc3f53e 100644
--- a/browser/locales/en-US/chrome/browser/browser.properties
+++ b/browser/locales/en-US/chrome/browser/browser.properties
@@ -169,6 +169,7 @@ safebrowsing.notAnAttackButton.accessKey=A
 # LOCALIZATION NOTE (privateBrowsingYesTitle, privateBrowsingNoTitle, privateBrowsingNeverAsk):
 # Access keys are specified by prefixing the desired letter with an ampersand.
 privateBrowsingDialogTitle=Start Private Browsing
+privateBrowsingMessageHeader=Would you like to start Private Browsing?
 privateBrowsingMessage=%S will save your current tabs for when you are done with your Private Browsing session.
 privateBrowsingYesTitle=&Start Private Browsing
 privateBrowsingNoTitle=&Cancel