Allowing less secure apps to access your account
Google may block sign in attempts from some apps or devices that do not use modern
security standards. Since these apps and devices are easier to break into, blocking them
helps keep your account safer.
Some examples of apps that do not support the latest security standards include:
...
Some Desktop mail clients like Microsoft Outlook and Mozilla Thunderbird
Therefore "Goozilla" considers Mozilla Thunderbird as less secure!?
Really "Goozilla"?
Ref.
https://support.google.com/accounts/answer/6010255