Comment 34 for bug 1187993

Also, per https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Modules#aside_%E2%80%94_.mjs_versus_.js, I think that we should consider either using the .js extension rather than .mjs or setting it to .js during a build step.

Reason: while editing the Apache config to add an AddType definition is not a huge deal, it might be just a bit too much of a friction point for an improvement that ideally would be backported down to the maintenance branches.