--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -13539,16 +13539,22 @@ nsDocShell::ConfirmRepost(bool* aRepost)
     return rv;
   }
 
   rv = appBundle->GetStringFromName("resendButton.label", button0Title);
   if (NS_FAILED(rv)) {
     return rv;
   }
 
+  // Make the repost prompt tab modal to prevent malicious pages from locking
+  // up the browser, see bug 1412559 for an example.
+  if (nsCOMPtr<nsIWritablePropertyBag2> promptBag = do_QueryInterface(prompter)) {
+    promptBag->SetPropertyAsBool(NS_LITERAL_STRING("allowTabModal"), true);
+  }
+
   int32_t buttonPressed;
   // The actual value here is irrelevant, but we can't pass an invalid
   // bool through XPConnect.
   bool checkState = false;
   rv = prompter->ConfirmEx(
     nullptr, msgString.get(),
     (nsIPrompt::BUTTON_POS_0 * nsIPrompt::BUTTON_TITLE_IS_STRING) +
       (nsIPrompt::BUTTON_POS_1 * nsIPrompt::BUTTON_TITLE_CANCEL),