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.