Call data stored to a variable, FirstName, in a "Correct" response to a submitted quiz answer

Hmmm. Typing the variable name within percentage signs should work to display a variable. So, first, double-check the variable name and the spelling.

Also, try using the INSERT > Reference button to put the variable within the text. That button allows you to select the variable to display. 

After all that, if the problem persists, I suggest you contact the Articulate staff. You can do that here: https://access.articulate.com/support/contact