A cookie editor that works for Firefox 57+ and with locally opened files (file:/// URLs).
What works:
* It displays the page's cookies and their values (so see other page's cookies, got to the other page's tab), in a popup when the add-on icon is clicked.
* Deleting individual cookies works.
What does not work:
* Creating individual cookies does not work, at least not for file URLs.
* Changing values did not work, at least not for file URLs.
