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The issue here seems to be that you want to run the javascript output by create-react-app on the build, but actually, you're just running a static file test.js which is in the src/main/app/public directory.

This test.js file is not touched by create-react-app at all, so you're right that it's not being compiled to ES5, it's simply being served statically as it is. The error you're seeing in Firefox is exactly because of that - it can't handle the import statements (fixing this directly is a separate issue which I won't get into here, but essentially you won't have this problem with the create-react-app compiled code because it'll be bundled and transpiled to ES5).

Only the code with the entry point at src/main/app/src/index.js will be compiled by create-react-app and indeed it is, and then as per your pom.xml configuration is then copied into target/classes/static/static/{bundle files}.js.

What you want to do is link to those create-react-app generated bundle files, instead of this test.js file. Whatever React code those built bundles contain will run just fine. It's unclear what the role of test.js is here, but to conclude, it's simply not being compiled at all and that's the cause of the error, nothing to do with React per se. You should just delete it and link instead to your create-react-app generated bundles.