This is because your form is set up with an action that points towards the signup link. A good way to do import contacts from Webflow forms to MailChimp is to use a Zapier integration. It’s really simple to set up, and works great!
You need to remove the action set in the form block.
To add first names, you can add an extra field to the form, then go to edit your Zapier integration and set it so that it pulls the first name from the new field you’ve added in and puts it in the right field in mailchimp.
Then make the first name and email input fields (and the button) as width:auto and then you can adjust as you wish using the bottom and top margins to space them out.