Closing the Thunderbird window can leave its mail session running intentionally. In Thunderbird 154 and later on Windows 10 or 11, the close-to-tray setting changes what the window's X button does. This particular feature is not available on macOS or Linux.

Check the configured behaviour first

Open Settings, General, System Integration and inspect the option for moving Thunderbird to the tray when closed. It is normally off, but users of the earlier minimise-to-tray setting are migrated to the newer close behaviour.

With the option enabled, X moves Thunderbird into the notification area and it continues receiving mail and notifications. Minimise now minimises the window normally. To fully quit from the tray, right-click the Thunderbird icon and choose Exit Thunderbird.

Use that distinction to describe the symptom accurately. “The window disappeared” and “I exited the application” are different observations. Repeatedly opening another window is not a useful first response if the existing session is simply hidden in the tray.

Rehearse a normal end-of-session routine

Before comparing the actions, save unfinished messages as drafts and finish any operation that needs attention. Note which setting is enabled. Use the window's close button, then inspect the notification area for Thunderbird instead of assuming the session ended.

Return to the application and confirm the saved draft is available without sending it. When ready to finish, use Exit Thunderbird from the tray. Check that its running tray icon disappears; if it persists, record that observation before resorting to a forced termination.

For someone preparing a computer handover, write down the intended routine in ordinary terms: save the draft, exit the application, and lock the computer when stepping away. Hiding a mail window should not be the handover's evidence that private work has been put away.

If ongoing background mail checks are unwanted, reconsider the close-to-tray setting and repeat the ordinary close test. Preserve the user's intended behaviour rather than enabling it merely because the option exists. When reporting a problem, include the Thunderbird version, Windows version, setting state and the exact button used. Those details separate this documented behaviour from a session that genuinely fails to exit.

Sources: Thunderbird documentation.