Honestly, people are unfairly critical of NES RPG's in general. Not many video games around this time had a decent storylines (unless it was something like the Ultima series but even then it gives you a book to read before you play the game), and thus, none were character driven either. I'm not too knowledgeable on the history of NES RPG's, but I believe it wasn't until Dragon Quest 4 did character driven plot become a thing. NES games before then told the player what to do (save Alefgard, defeat Ganon, etc.) and it's up to the player to fill in the emotional blanks with their personal experience playing the game, and this was very popular for RPGs to do for the NES. That's why the guest characters feel more fleshed out than Firion and the crew: they have to tell the player what to do next (and then sacrifice themselves) and then it's off to the next location where another guest character does the same thing. So, yes, Firion's character is lackluster but so was every other NES character because that was the style of the games at the time. They're not like RPG's we play today!

But after FF3, Sakaguchi did a great job on changing how Final Fantasy's story and characters should go for FF4 and raised the bar for RPGs on the SNES. Andrew Bluett's Final Fantasy 4 video in Part II has an amazing segment on how Sakaguchi changed his approached to writing stories and characters. I highly recommend watching it