Don't report firefox crash reports

tools/testing/dart/browser_controller.dart

Index: tools/testing/dart/browser_controller.dart
diff --git a/tools/testing/dart/browser_controller.dart b/tools/testing/dart/browser_controller.dart
index 2412d07895086ad980c8b1d1f7adec2ad1552f89..a6d061cf9bf248ff2b0506c5877135ec14badd37 100644
--- a/tools/testing/dart/browser_controller.dart
+++ b/tools/testing/dart/browser_controller.dart
@@ -744,7 +744,9 @@ class Firefox extends Browser {
         _cleanup = () { userDir.deleteSync(recursive: true); };
         var args = ["-profile", "${userDir.path}",
                     "-no-remote", "-new-instance", url];
-        return startBrowser(_binary, args);
+        var environment = new Map<String,String>.from(Platform.environment);
+        environment["MOZ_CRASHREPORTER_DISABLE"] = "1";
+        return startBrowser(_binary, args, environment: environment);
 
       });
     }).catchError((e) {