diff --git a/.travis.yml b/.travis.yml
index ea2e727d51..40bc7ddb9d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -56,10 +56,13 @@ script:
 ## (See #1082 for rationale).
 - TEST_PRODUCTION_MODE=1 npm run test-functional
 
+## run functional test suite in "npm legacy bundling mode"
+## (See #1639 for rationale).
+- TEST_PRODUCTION_MODE=1 TEST_LEGACY_BUNDLING=1 npm run test-functional
+
 ## NOTE: remove the custom python path from the npm config, and also
 ## remove production package.json.
 - npm config delete python
-- rm artifacts/production/package.json
 
 after_script: npm run publish-coverage
 
diff --git a/scripts/lib/config.js b/scripts/lib/config.js
index 331262a1f2..5c11b856d6 100644
--- a/scripts/lib/config.js
+++ b/scripts/lib/config.js
@@ -1,14 +1,13 @@
 module.exports = {
   clean: ['dist/*'],
   copy: {
-    productionModeArtifacts: {
+    productionModeAssets: {
       src: [
         'package.json',
         'package-lock.json',
         'dist/**',
         'bin/**',
       ],
-      dest: './artifacts/production',
     },
   },
   watch: {
diff --git a/scripts/test-functional b/scripts/test-functional
index 0fac1a29d0..86123c61f9 100755
--- a/scripts/test-functional
+++ b/scripts/test-functional
@@ -1,6 +1,9 @@
 #!/usr/bin/env node
 
+const path = require('path');
+
 const shell = require('shelljs');
+const tmp = require('tmp');
 
 const config = require('./lib/config');
 const {mochaFunctional} = require('./lib/mocha');
@@ -8,37 +11,40 @@ const {mochaFunctional} = require('./lib/mocha');
 shell.set('-e');
 
 const testProductionMode = process.env.TEST_PRODUCTION_MODE === '1';
+const testLegacyBundling = process.env.TEST_LEGACY_BUNDLING === '1';
 
-let execBuildOptions = {};
 let execMochaOptions = {};
 
+shell.exec('npm run build', testProductionMode ? {
+  env: {
+    ...process.env,
+    NODE_ENV: 'production',
+  },
+} : {});
+
 if (testProductionMode) {
-  execBuildOptions = {
-    env: {
-      ...process.env,
-      NODE_ENV: 'production',
-    },
-  };
+  const destDir = tmp.tmpNameSync();
+  const packageDir = tmp.tmpNameSync();
+  const npmInstallOptions = ['--production'];
+
+  if (testLegacyBundling) {
+    shell.echo('\nTest in "npm legacy bundling mode"');
+    npmInstallOptions.push('--legacy-bundling');
+  }
 
   execMochaOptions = {
     env: {
       ...process.env,
-      TEST_WEB_EXT_BIN: './artifacts/production/bin/web-ext',
+      TEST_WEB_EXT_BIN: path.join(destDir, 'node_modules', 'web-ext', 'bin', 'web-ext'),
     },
   };
-}
-
-shell.exec('npm run build', execBuildOptions);
 
-if (testProductionMode) {
   shell.echo('\nPreparing web-ext production mode environment...\n');
-  shell.rm('-rf', config.copy.productionModeArtifacts.dest);
-  shell.mkdir('-p', config.copy.productionModeArtifacts.dest);
-  shell.cp('-rf', config.copy.productionModeArtifacts.src,
-           config.copy.productionModeArtifacts.dest);
-
-  shell.pushd(config.copy.productionModeArtifacts.dest);
-  shell.exec('npm install --production');
+  shell.rm('-rf', destDir, packageDir);
+  shell.mkdir('-p', destDir, packageDir);
+  shell.cp('-rf', config.copy.productionModeAssets.src, packageDir);
+  shell.pushd(destDir);
+  shell.exec(`npm install ${npmInstallOptions.join(' ')} ${packageDir}`);
   shell.popd();
   shell.echo('\nProduction mode environment successfully created.\n');
 }