diff --git a/dom/tests/mochitest/fetch/sw_reroute.js b/dom/tests/mochitest/fetch/sw_reroute.js
index a4e10349c264..32d1246045bc 100644
--- a/dom/tests/mochitest/fetch/sw_reroute.js
+++ b/dom/tests/mochitest/fetch/sw_reroute.js
@@ -1,11 +1,13 @@
 var gRegistration;
+var iframe;
 
 function testScript(script) {
+  var scope = "./reroute.html?" + script.replace(".js", "");
   function setupSW(registration) {
     gRegistration = registration;
 
-    var iframe = document.createElement("iframe");
-    iframe.src = "reroute.html?" + script.replace(".js", "");
+    iframe = document.createElement("iframe");
+    iframe.src = scope;
     document.body.appendChild(iframe);
   }
 
@@ -16,13 +18,14 @@ function testScript(script) {
   }, function() {
     var scriptURL = location.href.includes("sw_empty_reroute.html")
                   ? "empty.js" : "reroute.js";
-    navigator.serviceWorker.register(scriptURL, {scope: "/"})
+    navigator.serviceWorker.register(scriptURL, {scope: scope})
       .then(swr => waitForState(swr.installing, 'activated', swr))
       .then(setupSW);
   });
 }
 
 function finishTest() {
+  iframe.remove();
   gRegistration.unregister().then(SimpleTest.finish, function(e) {
     dump("unregistration failed: " + e + "\n");
     SimpleTest.finish();