fix: firefoxPreview option should be set to mv3 only when explicitly requested (#2454)

Author: rpl

src/program.js

@@ -221,6 +221,10 @@ export class Program {
       throw new UsageError('Not enough arguments following: start-url');
     }
 
+    if (Array.isArray(argv.firefoxPreview) && !argv.firefoxPreview.length) {
+      argv.firefoxPreview = ['mv3'];
+    }
+
     return argv;
   }
 
@@ -692,9 +696,9 @@ Example: $0 --help run.
         type: 'array',
       },
       'firefox-preview': {
-        describe: 'Turn on developer preview features in Firefox',
+        describe: 'Turn on developer preview features in Firefox' +
+          ' (defaults to "mv3")',
         demandOption: false,
-        default: ['mv3'],
         type: 'array',
       },
       // Firefox for Android CLI options.

tests/unit/test.program.js

@@ -922,6 +922,18 @@ describe('program.main', () => {
     const {firefoxPreview} = fakeCommands.run.firstCall.args[0];
     assert.deepEqual(firefoxPreview, ['mv3']);
   });
+
+  it('does not set any firefox preview prefs by default', async () => {
+    const fakeCommands = fake(commands, {
+      run: () => Promise.resolve(),
+    });
+
+    await execProgram(['run'], {commands: fakeCommands});
+
+    const {firefoxPreview} = fakeCommands.run.firstCall.args[0];
+    assert.deepEqual(firefoxPreview, undefined);
+  });
+
 });
 
 describe('program.defaultVersionGetter', () => {