ES6: Why do I get this warning on code view

Hi, just curious, I got this error warning on code view:

‘let’ is available in ES6 (use ‘esversion: 6’) or Mozilla JS extensions (use moz)

The error is gone when I changed the declaration from let to var. The code works just fine and no error on dev console.

I just want to know what’s going on and anything about it that I should know.

Thanks in advance for any insight and clarification.