diff --git a/build-system/tasks/firebase.js b/build-system/tasks/firebase.js
index 8cca509841a5..b17a4c97b047 100644
--- a/build-system/tasks/firebase.js
+++ b/build-system/tasks/firebase.js
@@ -21,6 +21,7 @@ const path = require('path');
 const {build} = require('./build');
 const {clean} = require('./clean');
 const {dist} = require('./dist');
+const {enableLocalTesting} = require('./helpers');
 
 async function walk(dest) {
   const filelist = [];
@@ -47,16 +48,6 @@ async function copyAndReplaceUrls(src, dest) {
   await Promise.all(promises);
 }
 
-async function modifyThirdPartyUrl() {
-  const filePath = 'firebase/dist/amp.js';
-  const data = await fs.readFile('firebase/dist/amp.js', 'utf8');
-  const result = data.replace(
-    'self.AMP_CONFIG={',
-    'self.AMP_CONFIG={"thirdPartyUrl":location.origin,'
-  );
-  await fs.writeFile(filePath, result, 'utf8');
-}
-
 async function firebase() {
   if (!argv.nobuild) {
     await clean();
@@ -75,6 +66,7 @@ async function firebase() {
     });
     await replaceUrls('firebase/index.html');
   } else {
+    log(colors.green('Copying test/manual and examples folders.'));
     await Promise.all([
       copyAndReplaceUrls('test/manual', 'firebase/manual'),
       copyAndReplaceUrls('examples', 'firebase/examples'),
@@ -85,8 +77,15 @@ async function firebase() {
     fs.copy('dist', 'firebase/dist', {overwrite: true}),
     fs.copy('dist.3p/current', 'firebase/dist.3p/current', {overwrite: true}),
   ]);
+
+  if (argv.fortesting) {
+    await Promise.all([
+      enableLocalTesting('firebase/dist.3p/current/integration.js'),
+      enableLocalTesting('firebase/dist/amp.js'),
+    ]);
+  }
+
   await Promise.all([
-    modifyThirdPartyUrl(),
     fs.copyFile('firebase/dist/ww.max.js', 'firebase/dist/ww.js', {
       overwrite: true,
     }),
@@ -122,4 +121,6 @@ firebase.flags = {
   'file': 'File to deploy to firebase as index.html',
   'min': 'Source from minified files',
   'nobuild': 'Skips the gulp build|dist step.',
+  'fortesting':
+    'Expects an env var AMP_TESTING_HOST and writes this to AMP_CONFIG',
 };
diff --git a/contributing/TESTING.md b/contributing/TESTING.md
index cdd7c226d0f6..194b1c9cb1ec 100644
--- a/contributing/TESTING.md
+++ b/contributing/TESTING.md
@@ -337,6 +337,16 @@ After deploying, you can access your project publically at its hosting URL `http
 
 Additionally, you can create multiple projects and switch between them in the CLI using `firebase use your-project-name`.
 
+#### Testing Ads
+Testing ads in deployed demos requires whitelisting of 3p urls. You can do this by adding your intended deployment hostname as an environemnt variable `AMP_TESTING_HOST` and using the `fortesting` flag. For example:
+
+```
+export AMP_TESTING_HOST="my-project.firebaseapp.com"
+gulp firebase --fortesting
+firebase deploy
+```
+This will write "my-project.firebaseapp.com" as a third party url to relevant attributes in `AMP_CONFIG`, which is prepended to `amp.js` and `integration.js` files in the firebase folder. If you're curious about how this is done, feel free to inspect `build-system/tasks/firebase.js`.
+
 ## End-to-End Tests
 
 You can run and create E2E tests locally during development. Currently tests only run on Chrome, but support for additional browsers is underway. These tests have not been added to our CI build yet - but they will be added soon.