Some Mozilla Thunderbird users have noticed an issue where Thunderbird appears to freeze: it becomes unresponsive and will not start, even though it appears to be running. Thunderbird usually displays the error:

If Thunderbird is running but not responding, or if it indicates that your profile is in use, here are some things you can try.

Close and reopen Thunderbird. It is always worth closing and then reopening the application. Select Thunderbird > Quit Thunderbird from the menu and then reopen the application to see if this resolves the issue.

Quit Thunderbird on another computer. If Thunderbird is running with your profile on another computer, quit Thunderbird on that machine by selecting Thunderbird > Quit Thunderbird from the menu. Then log back in to the computer you are using.