Had also the same problem and saw on forums cas system errors (windows users) but still , fixing it like windows works for us too.
So if you have downloaded the game like it says in the previous posts and getting this CAS system error:
Download these files.https://cdn.discordapp.com/attachments/456860411682095124/457547475969114132/config.rarhttps://cdn.discordapp.com/attachments/376808947992100867/463974179642015744/FS735.wtf
•Extract the config.rar to data folder (you will need The Unarchiver app or something similar to do so)
•copy FS735.wtf to WTF folder
With this done you should be able to open the game through launcher or the Firestorm_mac file.
IF you get also the unable to connect error after trying to get in with your account do as Nenchew says:
In WTF/FS735.wtf change (with TextEdit)
SET portal "EU"
by
SET portal "legion.logon.firestorm-servers.com"