Initially promissing but just flat out stops working after a while. I add a cookie correctly for the domain (both dotted and not dotted, tried both ways), enter name and value, enter path '/', check the session checkbox. I expect the cookie to be sent to the domain with requests. It doesn't get sent. After a while it disappears, i.e. the search can no longer find it. Promising but broken.
EDIT to reply to the add-on's author's response:
I know when cookies can be refreshed. The site in question is one I am developing, and the cookie is neither interacted with on the client side, nor ever set by the server; it's just a flag to enable certain debugging functionality and it used to be a GET param but we decided to switch to a cookie and use an add-on like yours to set it whenever we need. Unfortunately I don't have time to debug your add-on, I simply need one that JustWorks™, and in fact I found one.
Nevertheless, I use this addon frequently and to my knowledge the use case you mention is functional... However, no project is immune to a bug and you should create an issue on the project submission so that we can better understand what you are trying to do. This platform is dedicated to managing bug reports and improvement proposals while addons-mozilla.org is dedicated notations.
Have you tried to visualize the status of your cookie via the Firefox development tools Ctrl+Shift+K, "Storage" tab?
"This is entirely up to the user agent, i.e. the user's browser, but usually, that will indeed be when the browser is closed" -- https://security.stackexchange.com/questions/33692/what-typically-is-the-expiration-date-of-a-session-cookie
"Cookies that 'expire at end of the session' expire unpredictably from the user's perspective!"
"If there is no expiry set on the cookie, then it is a session cookie and will live as long as the browser is open, and the sessionid is valid. If the server expires the authenticated sessions periodically, then the cookie will no longer be attached to a session on the server and will therefore be essentially null."
EDIT to reply to the Rulatir's response: Have you thought about clicking on the button "Save the current cookie" to finalize the creation of your new cookie? I only see this explanation.
I'm just asking for a full bug report describing the situation. As it stands I can only say that this addon works because I can not reproduce your approach on various sites I tested: The addon uses the Mozilla's APIs to create the cookies and the cookie is actually created in the database AND sent by GET requests with the other cookies.