--- a/toolkit/components/extensions/test/mochitest/test_ext_webrequest_filter.html
+++ b/toolkit/components/extensions/test/mochitest/test_ext_webrequest_filter.html
@@ -100,16 +100,18 @@ add_task(function* test_webRequest_filte
   yield extension.unload();
 });
 
 add_task(function* test_webRequest_filter_tab() {
   yield SpecialPowers.pushPrefEnv({
     set: [["dom.serviceWorkers.testing.enabled", true]],
   });
 
+  let img = `file_image_good.png?r=${Math.random()}`;
+
   let events = {
     "onBeforeRequest":     [{urls: ["<all_urls>"], tabId: windowData.tabId}],
     "onBeforeSendHeaders": [{urls: ["<all_urls>"], tabId: windowData.tabId}, ["requestHeaders"]],
     "onSendHeaders":       [{urls: ["<all_urls>"], tabId: windowData.tabId}, ["requestHeaders"]],
     "onBeforeRedirect":    [{urls: ["<all_urls>"], tabId: windowData.tabId}],
     "onHeadersReceived":   [{urls: ["<all_urls>"], tabId: windowData.tabId}, ["responseHeaders"]],
     "onResponseStarted":   [{urls: ["<all_urls>"], tabId: windowData.tabId}],
     "onCompleted":         [{urls: ["<all_urls>"], tabId: windowData.tabId}, ["responseHeaders"]],
@@ -125,26 +127,26 @@ add_task(function* test_webRequest_filte
 
   let extension = makeExtension(events);
   yield extension.startup();
   extension.sendMessage("set-expected", {expect, origin: location.href});
   yield extension.awaitMessage("continue");
 
   if (AppConstants.MOZ_BUILD_APP === "browser") {
     // We should not get events for a new window load.
-    let newWindow = window.open("file_image_good.png", "_blank", "width=100,height=100");
+    let newWindow = window.open(img, "_blank", "width=100,height=100");
     yield waitForLoad(newWindow);
     newWindow.close();
   }
 
   // We should not get background events.
   let registration = yield navigator.serviceWorker.register("webrequest_worker.js?test1", {scope: "."});
 
   // We should get events for the reload.
-  testWindow.location = "file_image_good.png";
+  testWindow.location = img;
   yield extension.awaitMessage("done");
 
   yield registration.unregister();
   yield extension.unload();
 });
 
 
 add_task(function* test_webRequest_filter_background() {