I didn't find how to do that rightaway, I had to do a little googling.
Here's how if you find yourself wondering around the same question:
- go to getpersonas.com
- create an account(if you don't already have one)
- then, on the details page
for any persona, in the top-right, just above the image will be a button
that says "Add to favorites" (if you are signed in).
- If you have the "Personas addon" installed, you'll be able to see the Personas you add to favorites listed, which is usefull if you, for instance, reinstall Firefox and want to keep its appearance the same: