Here's a thing you can do to quickly reveal any or all cookies set on your WebExtensions-compatible browser from any domain. For this example we'll use Chrome but it should work wherever you can debug a browser extension.
You need to have at least one add-on installed that has a background page; to find it, go to chrome://extensions
, be sure Developer Mode is on (top right sliding switch) and see if any of your installed extensions has a link to background page next to Inspect Views.
Once you find a link to a background page, click it. A Web inspector should appear; when it does, go to the Console tab and paste this:
chrome.cookies.getAll(
{domain: "facebook.com"},
results => {