Why does the request to the server not work only in firefox?

Question

JavaScript

fetch (whatwg-fetch polyfill) request works in all browsers (even IE) but fails in Firefox:

const headers = new Headers()
    headers.append('Content-Type', 'application/json')
    headers.append('user', '*****')
    const request = new Request('**********', {
        headers: headers
    })

    return window.fetch(request)
        .then(response => response.json())
        .then(result => {
            console.log(result)
        })
        .catch(e => {
            console.log('ошибка')
            console.log(e)
        })

It works in all browsers, but it does not work in firefox and in the console:

TypeError: NetworkError when attempting to fetch resource.

Header Access-Control-Allow-Origin * set
Also tried on axios, same situation