The Following Guide is the revision from the original guide with the corrected steps from the comments and myself.
This is a summarized guide I created while installing Firefly-III in a Proxmox (6.2-15) container running Ubuntu 20.04.
I followed this tutorial. It has a link to this other tutorial for installing mariaDB. Furthermore, the official Firefly-iii documentation is Here .
This guide has everything I did in one concise summary. Not much is explained. This will take about 1.5 hrs.