Update: A similar but more elegant solution can be found in the rollup-starter-code-splitting repo.
These browsers will use <script type=”module”> tags (and ignore <script nomodule>) but will throw when they encounter an import() call in the middle of your code.
This will display an error in the console on Firefox 60 (and the other browsers in the same situation) but should be transparent to the user.
You can see this approach implemented in the Rollup code-splitting example project.
If you have a better answer to this problem, please share it in the issues or here.