[asettico]

Firefox 86 introduced a new cookies management approach, so if you were logged in before the update, it's feasible that after that you could have found yourself logged out.
It's strange that suddenly the browser expires the session cookie.
Do you normally log out from the forum, or simply close the browser? When you check Remember me, it sets the expiration date of the cookie session a year later, so when the year expires, you are logged out.