Can not get cookie after set by using document.cookie on firefox 57

Hi, I’m writing a web extension and when I opened extension debugging mode through “about debugging”.

From console tab, I type “document.cookie = "username=tam;expires=Mon, 23 Feb 2018 17:35:38 GMT;path=/";” and Enter to set cookie. But after set, I type “document.cookie” it return empty string.

And my extension can not get cookie from document.cookie after set it.

I don’t know what happen with cookie that I set.

I have similar problem Not writing cookie in web_accessible_resources html page