How to get Thunderbird with MailMerge Plugin showing the Mail-List instead of each Adress in this List

I am using Thunderbird 140.5.0esr with MailMerge V11.2.0.
I created an AdressList to send Mass-Mails. But If I use the AdressList in To: the List is extracted by sending to each email.
When I place the AdressList in cc: which prevents showing adressees – used fields in Text Body e.g. FirstName will only been taken from the To Adress.
Lets say I use FirstName … all emails will get the FirstName from the to-Adress, but not from the emails in cc.
As well I got 290 sended mails by 290 Adresses …

What should I change to get
1.) the right FirstName in each email
2.) not to send 290 single mails.

Thanks
uwe