Getting 'ambiguous indirect export' error in JavaScript/TypeScript

I am getting this error: Syntax Error: ambiguous indirect export in Firefox. Sadly there is nothing to find at Google...

• This is my import:

<script type="module">
    import {someFunctionINeed} from "./js/functions.js";
    ...
</script>

• This is my export:

export function someFunctionINeed(cname) { ... }

• a part from my tsconfig.json:

"compilerOptions": {
    "module": "commonjs",
    "target": "es5",
    "sourceMap": true,
    "watch": true,
    "removeComments": true
}

• and something that's maybe relevant from the package.json:

"type": "module"

What did I miss?