Hi Lukas,
Unfortuately we do not official support the FontAwesome 5 icon pack yet as many icons have different classnames now.
However you are able to use Buttonizer and FA5 by just importing the stylesheet or javascript files into your theme. Some icons will work then from the list in the admin dashboard.
Most of the time there’s a different classname between light, normal and bold icons classnames. So not all icons are available from the list.
There is a cheatsheet available on fontawesome.com to convert old iconnames to FA5 (and keep using them) via javascript if I am right. I cannot gurantee that it fully covers all missing or changed classnames, but it is worth trying.
We do have plans to integrate and support FA5, but we’re working now on a new version of Buttonizer. After the new finished version of Buttonizer we will make sure to support more icon packs.
Let me know if this information helps you!
– Jasper