To personalize the welcome email with the recipient's first name from the Customers data extension, you use the Lookup function to retrieve the FirstName field based on the CustomerKey field.

Lookup Function: The Lookup function retrieves a value from a specified data extension.

Syntax:

%%=Lookup('Customers', 'FirstName', 'CustomerKey', CustomerKey)=%%

'Customers' is the name of the data extension from which to retrieve data.

'FirstName' is the field to retrieve.

'CustomerKey' is the field to match on.

CustomerKey is the value to match in the 'CustomerKey' field of the 'Customers' data extension.