It doesn't seem like there's a way to test an ES module from within an xpcshell-test. Test files aren't considered modules so get "import declarations may only appear at top level of a module" if you use `import`. Using `loadSubScript` on a modules leads to errors about `export` not being allowed outside a module. Example loadSubScript call that I wanted to change to import a module: https://dxr.mozilla.org/mozilla-central/rev/2318adaec61f07eccf66c3f742497aa973b15f2f/toolkit/components/payments/test/unit/test_PaymentsStore.js#4 Bug 1311728 would work but it would be non-standard.

The persons to ask on this topic would be :tcampbell and :kmag.