diff --git a/toolkit/mozapps/update/tests/unit_aus_update/remoteUpdateXML.js b/toolkit/mozapps/update/tests/unit_aus_update/remoteUpdateXML.js
index ed8be9a41eb3..8bff65880f38 100644
--- a/toolkit/mozapps/update/tests/unit_aus_update/remoteUpdateXML.js
+++ b/toolkit/mozapps/update/tests/unit_aus_update/remoteUpdateXML.js
@@ -14,6 +14,11 @@ function run_test() {
   start_httpserver();
   setUpdateURLOverride(gURLData + gHTTPHandlerPath);
   setUpdateChannel("test_channel");
+
+  // This test expects that the app.update.download.backgroundInterval
+  // preference doesn't already exist.
+  Services.prefs.deleteBranch("app.update.download.backgroundInterval");
+
   standardInit();
   do_execute_soon(run_test_pt01);
 }
diff --git a/toolkit/mozapps/update/tests/unit_aus_update/updateManagerXML.js b/toolkit/mozapps/update/tests/unit_aus_update/updateManagerXML.js
index 81076f6b0308..e464694553df 100644
--- a/toolkit/mozapps/update/tests/unit_aus_update/updateManagerXML.js
+++ b/toolkit/mozapps/update/tests/unit_aus_update/updateManagerXML.js
@@ -12,6 +12,10 @@ function run_test() {
 
   setUpdateChannel("test_channel");
 
+  // This test expects that the app.update.download.backgroundInterval
+  // preference doesn't already exist.
+  Services.prefs.deleteBranch("app.update.download.backgroundInterval");
+
   // XXXrstrong - not specifying a detailsURL will cause a leak due to bug 470244
   // and until bug 470244 is fixed this will not test the value for detailsURL
   // when it isn't specified in the update xml.