This issue appears in Firefox Developer Edition 53.0a2.
This take place in context of WebExtension development.
I have a lib script that export a simple variable.
my-lib.js
export const foo = 42
my-background-script.js
import foo from 'my-lib'
// ...
This throws a SyntaxError: import declarations may only appear at top level of a module
How can I import variables or functions declared in different javascript files ?