Thank you so much, Doublemyst, that did the trick. Initially I got a different error, an SSL error, but I was able to get rid of it by adding the password to the command, like I learned from Spaceinvader in his nextcloud tutorial.
GRANT ALL privileges ON `FIREFLYDBNAME-HERE`.* TO 'YOURFIREFLYUSER-HERE'@'%' IDENTIFIED BY 'PASSWORD-HERE';
Thanks a bunch. Looking forward to exploring the App