wxt/docs/guide/essentials/es-modules.md

ES Modules

Currently, ESM entrypoints are opt-in, so you must configure each entrypoint with that in mind.

HTML Pages

In general, you should always make HTML pages import ESM scripts, unless you need to support old browsers.

To make a script ESM, add type="module":

<script src="./main.ts"></script> <!-- [!code --] -->
<script src="./main.ts" type="module"></script> <!-- [!code ++] -->

Background

In your background script, set type: "module":

export default defineBackground({
  type: 'module', // [!code ++]
  main() {
    // ...
  },
});

:::warning Only MV3 supports ESM background scripts/service workers. When targeting MV2, the type option is ignored and the background is always bundled into a single file as IIFE. :::

Content Scripts

WXT does not yet include built-in support for ESM content scripts. The plan is to add support for chunking to reduce bundle size, but not support HMR for now. There are several technical issues that make implementing a generic solution for HMR impossible. See Content Script ESM Support #357 for details.

If you can't wait, and need ESM support right now, you can implement ESM support manually. See the ESM Content Script UI example to get started.