Cookies without an expiration timestamp are called "session cookies". [1] They should only be kept until the end of the browsing session.

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However, when Chrome or Firefox are configured to reopen tabs from last time upon start, they will keep session cookies when closing the browser. This even applies to tabs that were closed before shutting down the browser.

This is by design in Chrome Show archive.org snapshot and Firefox Show archive.org snapshot , has been like that for quite a while now and it seems they won't change their mind.

[1] We are not talking about the the Rails session cookie here, though it often is a session cookie.