Firefox Keeps Asking for Google Sign-In and 2FA

My dad has been using Linux Mint 22.3 for about a year, but over the past few months Firefox has started forgetting his Google and Gmail login. Every visit requires a full sign-in and 2FA, sometimes again only 20 minutes later. Firefox is not set to block cookies, and I cannot find any cookies for Gmail or other Google services in its storage list. Clearing cookies, restoring Firefox settings, and reinstalling the browser have not helped.

The problem began around the time he switched to a new phone running GrapheneOS, although his phone works normally and is often needed to approve the desktop sign-ins. Another user account on the same computer can stay logged into Gmail without any trouble, so the issue seems specific to his Firefox profile or account. Firefox has remained updated throughout. Has anyone seen this behavior, or know what could prevent Google's login session from being saved?