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diff --git a/modules/libpref/test/unit/test_stickyprefs.js b/modules/libpref/test/unit/test_stickyprefs.js
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+++ b/modules/libpref/test/unit/test_stickyprefs.js
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+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/licenses/publicdomain/  */
+
+const ps = Services.prefs;
+
+// A little helper to reset the service and load one pref file.
+function resetAndLoadDefaults() {
+  ps.resetPrefs();
+  ps.readDefaultPrefsFromFile(do_get_file("data/testPrefSticky.js"));
+}
+
+// A little helper to reset the service and load two pref files.
+function resetAndLoadAll() {
+  ps.resetPrefs();
+  ps.readDefaultPrefsFromFile(do_get_file("data/testPrefSticky.js"));
+  ps.readUserPrefsFromFile(do_get_file("data/testPrefStickyUser.js"));
+}
+
+// A little helper that saves the current state to a file in the profile
+// dir, then resets the service and re-reads the file it just saved.
+// Used to test what gets actually written - things the pref service decided
+// not to write don't exist at all after this call.
+function saveAndReload() {
+  let file = do_get_profile();
+  file.append("prefs.js");
+  ps.savePrefFile(file);
+
+  // Now reset the pref service and re-read what we saved.
+  ps.resetPrefs();
+
+  // Hack alert: on Windows nsLocalFile caches the size of savePrefFile from
+  // the .create() call above as 0. We call .exists() to reset the cache.
+  file.exists();
+
+  ps.readUserPrefsFromFile(file);
+}
+
+// A sticky pref should not be written if the value is unchanged.
+add_test(function notWrittenWhenUnchanged() {
+  resetAndLoadDefaults();
+  Assert.strictEqual(ps.getBoolPref("testPref.unsticky.bool"), true);
+  Assert.strictEqual(ps.getBoolPref("testPref.sticky.bool"), false);
+
+  // write prefs - but we haven't changed the sticky one, so it shouldn't be written.
+  saveAndReload();
+  // sticky should not have been written to the new file.
+  try {
+    ps.getBoolPref("testPref.sticky.bool");
+    Assert.ok(false, "expected failure reading this pref");
+  } catch (ex) {
+    Assert.ok(ex, "exception reading regular pref");
+  }
+  run_next_test();
+});
+
+// Loading a sticky `pref` then a `user_pref` for the same pref means it should
+// always be written.
+add_test(function writtenOnceLoadedWithoutChange() {
+  // Load the same pref file *as well as* a pref file that has a user_pref for
+  // our sticky with the default value. It should be re-written without us
+  // touching it.
+  resetAndLoadAll();
+  // reset and re-read what we just wrote - it should be written.
+  saveAndReload();
+  Assert.strictEqual(
+    ps.getBoolPref("testPref.sticky.bool"),
+    false,
+    "user_pref was written with default value"
+  );
+  run_next_test();
+});
+
+// If a sticky pref is explicicitly changed, even to the default, it is written.
+add_test(function writtenOnceLoadedWithChangeNonDefault() {
+  // Load the same pref file *as well as* a pref file that has a user_pref for
+  // our sticky - then change the pref. It should be written.
+  resetAndLoadAll();
+  // Set a new val and check we wrote it.
+  ps.setBoolPref("testPref.sticky.bool", false);
+  saveAndReload();
+  Assert.strictEqual(
+    ps.getBoolPref("testPref.sticky.bool"),
+    false,
+    "user_pref was written with custom value"
+  );
+  run_next_test();
+});
+
+// If a sticky pref is changed to the non-default value, it is written.
+add_test(function writtenOnceLoadedWithChangeNonDefault() {
+  // Load the same pref file *as well as* a pref file that has a user_pref for
+  // our sticky - then change the pref. It should be written.
+  resetAndLoadAll();
+  // Set a new val and check we wrote it.
+  ps.setBoolPref("testPref.sticky.bool", true);
+  saveAndReload();
+  Assert.strictEqual(
+    ps.getBoolPref("testPref.sticky.bool"),
+    true,
+    "user_pref was written with custom value"
+  );
+  run_next_test();
+});
+
+// Test that prefHasUserValue always returns true whenever there is a sticky
+// value, even when that value matches the default. This is mainly for
+// about:config semantics - prefs with a sticky value always remain bold and
+// always offer "reset" (which fully resets and drops the sticky value as if
+// the pref had never changed.)
+add_test(function hasUserValue() {
+  // sticky pref without user value.
+  resetAndLoadDefaults();
+  Assert.strictEqual(ps.getBoolPref("testPref.sticky.bool"), false);
+  Assert.ok(
+    !ps.prefHasUserValue("testPref.sticky.bool"),
+    "should not initially reflect a user value"
+  );
+
+  ps.setBoolPref("testPref.sticky.bool", false);
+  Assert.ok(
+    ps.prefHasUserValue("testPref.sticky.bool"),
+    "should reflect a user value after set to default"
+  );
+
+  ps.setBoolPref("testPref.sticky.bool", true);
+  Assert.ok(
+    ps.prefHasUserValue("testPref.sticky.bool"),
+    "should reflect a user value after change to non-default"
+  );
+
+  ps.clearUserPref("testPref.sticky.bool");
+  Assert.ok(
+    !ps.prefHasUserValue("testPref.sticky.bool"),
+    "should reset to no user value"
+  );
+  ps.setBoolPref("testPref.sticky.bool", false, "expected default");
+
+  // And make sure the pref immediately reflects a user value after load.
+  resetAndLoadAll();
+  Assert.strictEqual(ps.getBoolPref("testPref.sticky.bool"), false);
+  Assert.ok(
+    ps.prefHasUserValue("testPref.sticky.bool"),
+    "should have a user value when loaded value is the default"
+  );
+  run_next_test();
+});
+
+// Test that clearUserPref removes the "sticky" value.
+add_test(function clearUserPref() {
+  // load things such that we have a sticky value which is the same as the
+  // default.
+  resetAndLoadAll();
+  ps.clearUserPref("testPref.sticky.bool");
+
+  // Once we save prefs the sticky pref should no longer be written.
+  saveAndReload();
+  try {
+    ps.getBoolPref("testPref.sticky.bool");
+    Assert.ok(false, "expected failure reading this pref");
+  } catch (ex) {
+    Assert.ok(ex, "pref doesn't have a sticky value");
+  }
+  run_next_test();
+});
+
+// Test that a pref observer gets a notification fired when a sticky pref
+// has it's value changed to the same value as the default. The reason for
+// this behaviour is that later we might have other code that cares about a
+// pref being sticky (IOW, we notify due to the "state" of the pref changing
+// even if the value has not)
+add_test(function observerFires() {
+  // load things so there's no sticky value.
+  resetAndLoadDefaults();
+
+  function observe(subject, topic, data) {
+    Assert.equal(data, "testPref.sticky.bool");
+    ps.removeObserver("testPref.sticky.bool", observe);
+    run_next_test();
+  }
+  ps.addObserver("testPref.sticky.bool", observe);
+
+  ps.setBoolPref(
+    "testPref.sticky.bool",
+    ps.getBoolPref("testPref.sticky.bool")
+  );
+  // and the observer will fire triggering the next text.
+});