Normally, when closing Tor Browser, all cookies will be deleted and that's good. But is there a way to save a specific Tor cookie, maybe a login data of a website you don't want to enter again and again when using Tor Browser, so it can be used later?
Open Preferences and navigate to Privacy in Tor Browser by:
Click the drop-down menu next to "Tor Browser will:" and select "Use custom settings for history" and un-check "Always use private browsing mode". You will be asked to restart Tor browser. Click OK.
When the Tor browser restarts, navigate again to Preferences -> Privacy. Now you can change cookie settings like accept/block third-party cookies, set when to clear cookie, remember history, etc.
I would like to add to the previous response the following steps to save a cookie for future use. So, in that same window:
Assure that the following is true so that Tor privacy behavior doesn't deviate from its norm except for the websites of your choosing:
• "Remember my browsing and download history" is unchecked
• "Remember search and form history" is unchecked
• "Accept cookies from sites" is checked
• "Accept third-party cookies" is set to "Never"
• "Keep until" is set to "I close Tor Browser"
Click the "Exceptions..." button. Provide the address of the website of your choice, click "Allow" and then "Save Changes".
Click on the "Settings..." button. Under "History" unmark "Cookies", under "Data" unmark both "Site Preferences" and "Offline Website Data". Click "OK".
In conclusion, allow me to explain the necessity of these settings. By unmarking "Site Preferences" you allowed Tor to save the exception you made for the website of your choice. By unmchecking "Offline Website Data" you allowed Tor to save offline data such as cookies, wich by now, is the only type of offline date you've allowed to save. Undesired cookies from other websites should still be erased, since "Keep until" is set to "I close Tor Browser". You could check that yourself, by visiting any given site to have its cookies added in the list of installed cookies in "Preferences/Privacy/Show Cookies..." and then restarting the browser and seeing if they are still there. And they should NOT, given that the above configuration was met.