Great to hear that executeScript | document.cookie = \"username=John Doe\";" usually works.
But executeScript_Sandbox blocks all website access of the Javascript to avoid content security errors. So no surprise that it does not work as workaround for Github.
Can you maybe manually copy the cookie text file to the browser cache? Does this extension work with Github?
EditThisCookie can indeed manipulate cookies on GitHub, so it’s possible to manipulate cookies through an extension.
Luckily, the website I’m currently dealing with it has no JS restriction other than an httpOnly flag that prevents cookies from being manipulated. I can deleteAllCookies and then recreate them.
However, for the future it might be wise to look at implementing cookie-related commands (particularly get/set)