diff --git a/js/xpconnect/tests/chrome/test_bug361111.xul b/js/xpconnect/tests/chrome/test_bug361111.xul
index a3f25fc381c..4b790ea2d2b 100644
--- a/js/xpconnect/tests/chrome/test_bug361111.xul
+++ b/js/xpconnect/tests/chrome/test_bug361111.xul
@@ -22,7 +22,12 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=361111
SimpleTest.waitForExplicitFinish();
document.documentElement.setAttribute("onclick", "%");
is(1, 1, "Good, setting a bogus onclick did not throw.");
- SimpleTest.finish();
+
+ // Bonus test - make sure that flushPrefEnv is appropriately
+ // called at the end of the test. It would be nice if there were
+ // somewhere in the harness that this could live, but there isn't.
+ SpecialPowers.pushPrefEnv({set: [['testing.some_arbitrary_pref', true]]},
+ function() { SimpleTest.finish(); });
]]>
diff --git a/js/xpconnect/tests/chrome/test_bug448587.xul b/js/xpconnect/tests/chrome/test_bug448587.xul
index db5487deeb5..1b173556d2a 100644
--- a/js/xpconnect/tests/chrome/test_bug448587.xul
+++ b/js/xpconnect/tests/chrome/test_bug448587.xul
@@ -19,6 +19,12 @@ https://bugzilla.mozilla.org/show_bug.cgi?id=448587.xul
diff --git a/testing/mochitest/tests/test_SpecialPowersExtension2.html b/testing/mochitest/tests/test_SpecialPowersExtension2.html
index ac2a7682149..fc77aa82a5b 100644
--- a/testing/mochitest/tests/test_SpecialPowersExtension2.html
+++ b/testing/mochitest/tests/test_SpecialPowersExtension2.html
@@ -11,6 +11,8 @@