diff --git a/src/cmd/build.js b/src/cmd/build.js
index 1b28ea3f38..3f365985fb 100644
--- a/src/cmd/build.js
+++ b/src/cmd/build.js
@@ -31,8 +31,9 @@ function defaultPackageCreator(
           filter: (...args) => fileFilter.wantFile(...args),
         })
         .then((buffer) => {
+          let extension = 'zip';
           let packageName = safeFileName(
-            `${manifestData.name}-${manifestData.version}.xpi`);
+            `${manifestData.name}-${manifestData.version}.${extension}`);
           let extensionPath = path.join(artifactsDir, packageName);
           let stream = createWriteStream(extensionPath);
           let promisedStream = streamToPromise(stream);
diff --git a/tests/fixtures/minimal_extension-1.0.xpi b/tests/fixtures/minimal_extension-1.0.zip
similarity index 100%
rename from tests/fixtures/minimal_extension-1.0.xpi
rename to tests/fixtures/minimal_extension-1.0.zip
diff --git a/tests/test-cmd/test.build.js b/tests/test-cmd/test.build.js
index 46c80510a3..77387282a8 100644
--- a/tests/test-cmd/test.build.js
+++ b/tests/test-cmd/test.build.js
@@ -24,7 +24,7 @@ describe('build', () => {
         })
         .then((buildResult) => {
           assert.match(buildResult.extensionPath,
-                       /minimal_extension-1\.0\.xpi$/);
+                       /minimal_extension-1\.0\.zip$/);
           return buildResult.extensionPath;
         })
         .then((extensionPath) => zipFile.open(extensionPath))
@@ -62,12 +62,12 @@ describe('build', () => {
       })
       .then((buildResult) => {
         assert.match(buildResult.extensionPath,
-                     /the_extension-0\.0\.1\.xpi$/);
+                     /the_extension-0\.0\.1\.zip$/);
         return buildResult.extensionPath;
       })
   ));
 
-  it('asks FileFilter what files to include in the XPI', () => {
+  it('asks FileFilter what files to include in the ZIP', () => {
     let zipFile = new ZipFile();
     let fileFilter = new FileFilter({
       filesToIgnore: ['**/background-script.js'],
@@ -99,7 +99,7 @@ describe('build', () => {
         {manifestData: basicManifest, onSourceChange, fileFilter})
         .then((buildResult) => {
           // Make sure we still have a build result.
-          assert.match(buildResult.extensionPath, /\.xpi$/);
+          assert.match(buildResult.extensionPath, /\.zip$/);
           return buildResult;
         })
         .then((buildResult) => {
@@ -122,7 +122,7 @@ describe('build', () => {
             .then(() => args.onChange());
         })
         .then((buildResult) => {
-          assert.match(buildResult.extensionPath, /\.xpi$/);
+          assert.match(buildResult.extensionPath, /\.zip$/);
           return fs.stat(buildResult.extensionPath);
         })
         .then((stat) => {
@@ -209,7 +209,7 @@ describe('build', () => {
       assert.equal(filter.wantFile('some.xpi'), true);
       assert.equal(filter.wantFile('manifest.json'), false);
     });
-    
+
     it('ignores node_modules by default', () => {
       assert.equal(defaultFilter.wantFile('path/to/node_modules'), false);
     });
diff --git a/tests/test-cmd/test.sign.js b/tests/test-cmd/test.sign.js
index 0655605eda..4dd054a65a 100644
--- a/tests/test-cmd/test.sign.js
+++ b/tests/test-cmd/test.sign.js
@@ -75,7 +75,7 @@ describe('sign', () => {
           // Do a sanity check that a built extension was passed to the
           // signer.
           assert.include(stubs.signAddon.firstCall.args[0].xpiPath,
-                         'minimal_extension-1.0.xpi');
+                         'minimal_extension-1.0.zip');
         });
     }
   ));
diff --git a/tests/test-firefox/test.firefox.js b/tests/test-firefox/test.firefox.js
index 4a6cfd1236..2f3a90259a 100644
--- a/tests/test-firefox/test.firefox.js
+++ b/tests/test-firefox/test.firefox.js
@@ -354,7 +354,7 @@ describe('firefox', () => {
       return withTempDir(
         (tmpDir) => {
           let data = {
-            extensionPath: fixturePath('minimal_extension-1.0.xpi'),
+            extensionPath: fixturePath('minimal_extension-1.0.zip'),
             profile: undefined,
             profileDir: path.join(tmpDir.path(), 'profile'),
           };
@@ -411,19 +411,19 @@ describe('firefox', () => {
 
     it('requires a directory path for proxy installs', () => setUp(
       (data) => {
-        const xpiPath = fixturePath('minimal_extension-1.0.xpi');
+        const extensionPath = fixturePath('minimal_extension-1.0.zip');
         return firefox.installExtension(
           {
             manifestData: basicManifest,
             profile: data.profile,
-            extensionPath: xpiPath,
+            extensionPath: extensionPath,
             asProxy: true,
           })
           .then(makeSureItFails())
           .catch(onlyInstancesOf(WebExtError, (error) => {
             assert.match(error.message,
                          /must be the extension source directory/);
-            assert.include(error.message, xpiPath);
+            assert.include(error.message, extensionPath);
           }));
       }
     ));