It's puzzling when you ask "[https://emailbackupwizard.com/blog/why-does-my-gmail-automatically-sign-out/ Why does my Gmail automatically sign out?]" after setting it up in Thunderbird, especially since desktop email clients usually maintain a continuous connection. The message "You have been signed out of this account. This may have happened automatically because another user signed in from the same browser" strongly suggests a conflict, but it's typically a browser-related error, not a direct Thunderbird logout. The most likely culprit for Thunderbird losing access to your Gmail is an authentication issue. Modern Gmail access in Thunderbird relies on OAuth2, a secure protocol. If the OAuth2 token becomes invalid or expires, or if Thunderbird's internal web component (used for the OAuth2 login pop-up) has issues like cookies being disabled, it can repeatedly prompt you for login. Less common, but possible, are: Google's security systems detecting unusual activity and temporarily blocking access, or, if you're not using OAuth2 and have 2-Step Verification enabled, an expired or revoked App Password. While less direct, a concurrent, conflicting sign-in in a web browser using the same Google account could also contribute to Google invalidating sessions. To resolve this, ensure your Thunderbird Gmail account is configured to use OAuth2 for both incoming (IMAP) and outgoing (SMTP) server settings. Also, verify that Thunderbird is allowed to accept cookies under its Privacy & Security settings, as this is crucial for the OAuth2 process. Removing saved passwords for the Gmail account in Thunderbird and then restarting it can force a fresh OAuth2 re-authentication.