Pass variable in the field label

I need to pass in someones first name, which I know I can do using variables in the URL. How can I then use one of those variables in the label for a question? as an example, I pass in the first name of someone and I want the question to say "[first name], how likely are you to recommend us to your friend?

You can add the variable directly to the label.

Example: If the full name variable is {name}, to get the first name use {name:first}

Result:

Yes, how do I actually set it up so it says Hi John?

You need to use the field name, this guide will help you finding it: https://www.jotform.com/help/146-How-to-Find-Field-IDs-and-Names 

Once you have the field name, you can add it to the field label where you want to include the value.