diff --git a/extensions/cookie/test/unit/head_cookies.js b/extensions/cookie/test/unit/head_cookies.js
index ded1a9f2f6c0..8647a446b4a5 100644
--- a/extensions/cookie/test/unit/head_cookies.js
+++ b/extensions/cookie/test/unit/head_cookies.js
@@ -36,6 +36,26 @@ function do_check_throws(f, result, stack)
   do_throw("expected result " + result + ", none thrown", stack);
 }
 
+// Helper to step a generator function and catch a StopIteration exception.
+function do_run_generator(generator)
+{
+  try {
+    generator.next();
+  } catch (e) {
+    if (e != StopIteration)
+      do_throw("caught exception " + e, Components.stack.caller);
+  }
+}
+
+// Helper to finish a generator function test.
+function do_finish_generator_test(generator)
+{
+  do_execute_soon(function() {
+    generator.close();
+    do_test_finished();
+  });
+}
+
 function _observer(generator, service, topic) {
   Services.obs.addObserver(this, topic, false);
 
@@ -51,14 +71,8 @@ _observer.prototype = {
     Services.obs.removeObserver(this, this.topic);
 
     // Continue executing the generator function.
-    if (this.generator) {
-      try {
-        this.generator.next();
-      } catch (e) {
-        if (e != StopIteration)
-          do_throw("caught exception " + e, Components.stack.caller);
-      }
-    }
+    if (this.generator)
+      do_run_generator(this.generator);
 
     this.generator = null;
     this.service = null;
diff --git a/extensions/cookie/test/unit/test_eviction.js b/extensions/cookie/test/unit/test_eviction.js
index 3787cafbbef8..bb9f66e9f51c 100644
--- a/extensions/cookie/test/unit/test_eviction.js
+++ b/extensions/cookie/test/unit/test_eviction.js
@@ -4,105 +4,194 @@
 
 let test_generator = do_run_test();
 
-function run_test() {
+function run_test()
+{
   do_test_pending();
-  test_generator.next();
+  do_run_generator(test_generator);
 }
 
-function finish_test() {
+function continue_test()
+{
+  do_run_generator(test_generator);
+}
+
+function repeat_test()
+{
+  // The test is probably going to fail because setting a batch of cookies took
+  // a significant fraction of 'gPurgeAge'. Compensate by rerunning the
+  // test with a larger purge age.
+  do_check_true(gPurgeAge < 64);
+  gPurgeAge *= 2;
+
   do_execute_soon(function() {
     test_generator.close();
-    do_test_finished();
+    test_generator = do_run_test();
+    do_run_generator(test_generator);
   });
 }
 
+// Purge threshold, in seconds.
+let gPurgeAge = 1;
+
+// Required delay to ensure a purge occurs, in milliseconds. This must be at
+// least gPurgeAge + 10%, and includes a little fuzz to account for timer
+// resolution and possible differences between PR_Now() and Date.now().
+function get_purge_delay()
+{
+  return gPurgeAge * 1100 + 100;
+}
+
 function do_run_test()
 {
   // Set up a profile.
   let profile = do_get_profile();
 
   // twiddle prefs to convenient values for this test
-  Services.prefs.setIntPref("network.cookie.purgeAge", 1);
-  Services.prefs.setIntPref("network.cookie.maxNumber", 1000);
+  Services.prefs.setIntPref("network.cookie.purgeAge", gPurgeAge);
+  Services.prefs.setIntPref("network.cookie.maxNumber", 100);
+
+  let expiry = Date.now() / 1000 + 1000;
 
   // eviction is performed based on two limits: when the total number of cookies
-  // exceeds maxNumber + 10% (1100), and when cookies are older than purgeAge
+  // exceeds maxNumber + 10% (110), and when cookies are older than purgeAge
   // (1 second). purging is done when both conditions are satisfied, and only
   // those cookies are purged.
 
   // we test the following cases of eviction:
   // 1) excess and age are satisfied, but only some of the excess are old enough
   // to be purged.
-  force_eviction(1101, 50);
+  Services.cookiemgr.removeAll();
+  if (!set_cookies(0, 5, expiry)) {
+    repeat_test();
+    return;
+  }
+  // Sleep a while, to make sure the first batch of cookies is older than
+  // the second (timer resolution varies on different platforms).
+  do_timeout(get_purge_delay(), continue_test);
+  yield;
+  if (!set_cookies(5, 111, expiry)) {
+    repeat_test();
+    return;
+  }
 
   // Fake a profile change, to ensure eviction affects the database correctly.
   do_close_profile(test_generator);
   yield;
   do_load_profile();
-
-  do_check_true(check_remaining_cookies(1101, 50, 1051));
+  do_check_true(check_remaining_cookies(111, 5, 106));
 
   // 2) excess and age are satisfied, and all of the excess are old enough
   // to be purged.
-  force_eviction(1101, 100);
+  Services.cookiemgr.removeAll();
+  if (!set_cookies(0, 10, expiry)) {
+    repeat_test();
+    return;
+  }
+  do_timeout(get_purge_delay(), continue_test);
+  yield;
+  if (!set_cookies(10, 111, expiry)) {
+    repeat_test();
+    return;
+  }
+
   do_close_profile(test_generator);
   yield;
   do_load_profile();
-  do_check_true(check_remaining_cookies(1101, 100, 1001));
+  do_check_true(check_remaining_cookies(111, 10, 101));
 
   // 3) excess and age are satisfied, and more than the excess are old enough
   // to be purged.
-  force_eviction(1101, 500);
+  Services.cookiemgr.removeAll();
+  if (!set_cookies(0, 50, expiry)) {
+    repeat_test();
+    return;
+  }
+  do_timeout(get_purge_delay(), continue_test);
+  yield;
+  if (!set_cookies(50, 111, expiry)) {
+    repeat_test();
+    return;
+  }
+
   do_close_profile(test_generator);
   yield;
   do_load_profile();
-  do_check_true(check_remaining_cookies(1101, 500, 1001));
+  do_check_true(check_remaining_cookies(111, 50, 101));
 
   // 4) excess but not age are satisfied.
-  force_eviction(2000, 0);
+  Services.cookiemgr.removeAll();
+  if (!set_cookies(0, 120, expiry)) {
+    repeat_test();
+    return;
+  }
+
   do_close_profile(test_generator);
   yield;
   do_load_profile();
-  do_check_true(check_remaining_cookies(2000, 0, 2000));
+  do_check_true(check_remaining_cookies(120, 0, 120));
 
   // 5) age but not excess are satisfied.
-  force_eviction(1100, 200);
+  Services.cookiemgr.removeAll();
+  if (!set_cookies(0, 20, expiry)) {
+    repeat_test();
+    return;
+  }
+  do_timeout(get_purge_delay(), continue_test);
+  yield;
+  if (!set_cookies(20, 110, expiry)) {
+    repeat_test();
+    return;
+  }
+
   do_close_profile(test_generator);
   yield;
   do_load_profile();
-  do_check_true(check_remaining_cookies(1100, 200, 1100));
+  do_check_true(check_remaining_cookies(110, 20, 110));
 
-  finish_test();
+  do_finish_generator_test(test_generator);
 }
 
-// Set 'aNumberTotal' cookies, ensuring that the first 'aNumberOld' cookies
-// will be measurably older than the rest.
-function
-force_eviction(aNumberTotal, aNumberOld)
+// Set 'end - begin' total cookies, with consecutively increasing hosts numbered
+// 'begin' to 'end'.
+function set_cookies(begin, end, expiry)
 {
-  Services.cookiemgr.removeAll();
-  var expiry = (Date.now() + 1e6) * 1000;
+  do_check_true(begin != end);
 
-  var i;
-  for (i = 0; i < aNumberTotal; ++i) {
-    var host = "eviction." + i + ".tests";
+  let beginTime;
+  for (let i = begin; i < end; ++i) {
+    let host = "eviction." + i + ".tests";
     Services.cookiemgr.add(host, "", "test", "eviction", false, false, false,
       expiry);
 
-    if (i == aNumberOld - 1) {
-      // Sleep a while, to make sure the first batch of cookies is older than
-      // the second (timer resolution varies on different platforms). This
-      // delay must be measurably greater than the eviction age threshold.
-      sleep(1100);
-    }
+    if (i == begin)
+      beginTime = get_creationTime(i);
   }
+
+  let endTime = get_creationTime(end - 1);
+  do_check_true(endTime > beginTime);
+  if (endTime - beginTime > gPurgeAge * 1000000) {
+    // Setting cookies took an amount of time very close to the purge threshold.
+    // Retry the test with a larger threshold.
+    return false;
+  }
+
+  return true;
+}
+
+function get_creationTime(i)
+{
+  let host = "eviction." + i + ".tests";
+  let enumerator = Services.cookiemgr.getCookiesFromHost(host);
+  do_check_true(enumerator.hasMoreElements());
+  let cookie = enumerator.getNext().QueryInterface(Ci.nsICookie2);
+  return cookie.creationTime;
 }
 
 // Test that 'aNumberToExpect' cookies remain after purging is complete, and
 // that the cookies that remain consist of the set expected given the number of
 // of older and newer cookies -- eviction should occur by order of lastAccessed
 // time, if both the limit on total cookies (maxNumber + 10%) and the purge age
-// are exceeded.
+// + 10% are exceeded.
 function check_remaining_cookies(aNumberTotal, aNumberOld, aNumberToExpect) {
   var enumerator = Services.cookiemgr.enumerator;
 
@@ -120,11 +209,3 @@ function check_remaining_cookies(aNumberTotal, aNumberOld, aNumberToExpect) {
 
   return i == aNumberToExpect;
 }
-
-// delay for a number of milliseconds
-function sleep(delay)
-{
-  var start = Date.now();
-  while (Date.now() < start + delay);
-}
-