--- a/toolkit/modules/tests/browser/browser_WebRequest_ancestors.js
+++ b/toolkit/modules/tests/browser/browser_WebRequest_ancestors.js
@@ -10,19 +10,20 @@ var {PromiseUtils} = Cu.import("resource
 add_task(async function test_ancestors_exist() {
   let deferred = PromiseUtils.defer();
   function onBeforeRequest(details) {
     info(`onBeforeRequest ${details.url}`);
     ok(typeof details.frameAncestors === "object", `ancestors exists [${typeof details.frameAncestors}]`);
     deferred.resolve();
   }
 
-  WebRequest.onBeforeRequest.addListener(onBeforeRequest, {urls: new MatchPatternSet(["http://mochi.test/*"])}, ["blocking"]);
+  // Filter on a path to ensure the root favicon request doesn't confuse the test.
+  WebRequest.onBeforeRequest.addListener(onBeforeRequest, {urls: new MatchPatternSet(["http://mochi.test/test/*"])}, ["blocking"]);
 
-  let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser, "http://mochi.test:8888/");
+  let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser, "http://mochi.test:8888/test/");
   await deferred.promise;
   await BrowserTestUtils.removeTab(tab);
 
   WebRequest.onBeforeRequest.removeListener(onBeforeRequest);
 });
 
 add_task(async function test_ancestors_null() {
   let deferred = PromiseUtils.defer();
@@ -42,13 +43,13 @@ add_task(async function test_ancestors_n
       xhr.onload = () => { resolve(xhr.responseText); };
       xhr.onerror = () => { reject(xhr.status); };
       // use a different contextId to avoid auth cache.
       xhr.setOriginAttributes({userContextId: 1});
       xhr.send();
     });
   }
 
-  await fetch("http://mochi.test:8888/");
+  await fetch("http://mochi.test:8888/test/");
   await deferred.promise;
 
   WebRequest.onBeforeRequest.removeListener(onBeforeRequest);
 });