When sending mass email to Users with an email template, the merge field value shows the sender name instead of the recipient name (User's name). Sending mass email to Users applies to Salesforce Classic ONLY.
Merge field {!User.FirstName} is used to indicate the name of the User that is currently logged in prior to send the mass email. To refer to the recipient, make sure you use the correct merge field which is {!Receiving_User.Name}.
In the Available Merge Field section where you edit to add a merge field value in a Classic Email template:
For example:
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Dear {!Receiving_User.FirstName},
Welcome to Salesforce! This is just a test.
Best regards,
Salesforce Team
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000380608

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