Check the box associated to I understand the risk and click on Create Account button.
Alternatively, switch to mail service provider that offers a secure connection such as Yahoo, Gmail, etc.
To verify if the problem is being caused by a specific add-on, try comparing application behaviour in Normal mode to that of the behaviour in Safe mode.
An email address that forwards messages to another email account is referred to as the alias.
Mozilla Thunderbird was originally launched as Minotaur, shortly after Phoenix (the original name for Mozilla Firefox); the project failed to gain momentum.
In the Safe mode, you can reset Thunderbird settings or disable the add-ons that are creating issues.If it does then you may require changing Thunderbird settings.