diff -r 000000000000 -r 6474c204b198 modules/libpref/test/unit/test_bug790374.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/modules/libpref/test/unit/test_bug790374.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,55 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/licenses/publicdomain/  */
+
+function run_test() {
+  const Cc = Components.classes;
+  const Ci = Components.interfaces;
+  const Cr = Components.results;
+  const PREF_NAME = "testPref";
+
+  var ps = Cc["@mozilla.org/preferences-service;1"]
+           .getService(Ci.nsIPrefService);
+  var prefs = ps.getDefaultBranch(null);
+  var userprefs = ps.getBranch(null);
+
+  /* First, test to make sure we can parse a float from a string properly. */
+  prefs.setCharPref(PREF_NAME, "9.674");
+  prefs.lockPref(PREF_NAME);
+  var myFloat = 9.674;
+  var fudge = 0.001;
+  var floatPref = userprefs.getFloatPref(PREF_NAME);
+  do_check_true(myFloat+fudge >= floatPref);
+  do_check_true(myFloat-fudge <= floatPref);
+
+  /* Now test some failure conditions. */
+  prefs.unlockPref(PREF_NAME);
+  prefs.setCharPref(PREF_NAME, "");
+  prefs.lockPref(PREF_NAME);
+  do_check_throws(function() {
+    userprefs.getFloatPref(PREF_NAME);
+  }, Cr.NS_ERROR_ILLEGAL_VALUE);
+
+  prefs.unlockPref(PREF_NAME);
+  prefs.setCharPref(PREF_NAME, "18.0a1");
+  prefs.lockPref(PREF_NAME);
+
+  do_check_throws(function() {
+    userprefs.getFloatPref(PREF_NAME);
+  }, Cr.NS_ERROR_ILLEGAL_VALUE);
+
+  prefs.unlockPref(PREF_NAME);
+  prefs.setCharPref(PREF_NAME, "09.25.2012");
+  prefs.lockPref(PREF_NAME);
+
+  do_check_throws(function() {
+    userprefs.getFloatPref(PREF_NAME);
+  }, Cr.NS_ERROR_ILLEGAL_VALUE);
+
+  prefs.unlockPref(PREF_NAME);
+  prefs.setCharPref(PREF_NAME, "aString");
+  prefs.lockPref(PREF_NAME);
+
+  do_check_throws(function() {
+    userprefs.getFloatPref(PREF_NAME);
+  }, Cr.NS_ERROR_ILLEGAL_VALUE);
+}