TIL it’s possible to put certain domains on an allow-list in Firefox, which stops them from being automatically deleted when the browser is closed (if you have that option turned on). Handy if you have some sites where you don’t mind a session persisting, but want the default for all other cookies to be “only persist as long as the browser session.”

screenshot of Firefox Cookies and Site Data options, with a callout pointing to 'Manage Permissions'