And i mean auto spell check to be precise. :)

Automatically applied Signatures are set in Account Settings. either in the 'Signature Text' text box or attached via a pre-made html file so they can auto added to email when you click on 'Write'.

Signature html files can be created in the Thunderbird Write window and then saved as a file of type html. When you are creating the signature in a Write window, spell check will work on everything you type. Thus once created and checked and saved as html file and then attached for use in Account Settings for the mail account, you never need to check the signature again.

If signatures are never pre added and manually typed every time at the end of a message then they will be checked because it is part of the message and not an appended file. So if you accidentally typed 'Regaarrds' instead of 'Regards' then it would flag as needing checking.

Menu app icon > Options > Composition > 'Spelling' tab This is where you can set the default settings for spell check.

In a Write message, you can set it here: Options > select 'Spell check as you type'