diff -r 000000000000 -r 6474c204b198 toolkit/devtools/tests/unit/test_safeErrorString.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/devtools/tests/unit/test_safeErrorString.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,57 @@
+/* -*- Mode: js; js-indent-level: 2; -*- */
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// Test DevToolsUtils.safeErrorString
+
+function run_test() {
+  test_with_error();
+  test_with_tricky_error();
+  test_with_string();
+  test_with_thrower();
+  test_with_psychotic();
+}
+
+function test_with_error() {
+  let s = DevToolsUtils.safeErrorString(new Error("foo bar"));
+  // Got the message.
+  do_check_true(s.contains("foo bar"));
+  // Got the stack.
+  do_check_true(s.contains("test_with_error"))
+  do_check_true(s.contains("test_safeErrorString.js"));
+  // Got the lineNumber and columnNumber.
+  do_check_true(s.contains("Line"));
+  do_check_true(s.contains("column"));
+}
+
+function test_with_tricky_error() {
+  let e = new Error("batman");
+  e.stack = { toString: Object.create(null) };
+  let s = DevToolsUtils.safeErrorString(e);
+  // Still got the message, despite a bad stack property.
+  do_check_true(s.contains("batman"));
+}
+
+function test_with_string() {
+  let s = DevToolsUtils.safeErrorString("not really an error");
+  // Still get the message.
+  do_check_true(s.contains("not really an error"));
+}
+
+function test_with_thrower() {
+  let s = DevToolsUtils.safeErrorString({
+    toString: () => {
+      throw new Error("Muahahaha");
+    }
+  });
+  // Still don't fail, get string back.
+  do_check_eq(typeof s, "string");
+}
+
+function test_with_psychotic() {
+  let s = DevToolsUtils.safeErrorString({
+    toString: () => Object.create(null)
+  });
+  // Still get a string out, and no exceptions thrown
+  do_check_eq(typeof s, "string");
+}