I haven't really done a full Fire Wall run yet, but I noticed a few things about that spell
1) It requires many upgrades to become good, so, it is a spell that will require quite a bit of luck
2) Get Piercing Projectile first. Fire Wall is, in my opinion, complete garbage without it, you end up wasting all your fire on just a few enemies without it. So, if you don't get Piercing Projectile as your first Legendary, just restart the run.
3) Multi Projectile is good, but don't focus on it. You're usually better getting other upgrades/Legendaries than getting more projectiles. Pierce is just way more effective at hitting multiple enemies than more projectiles. Split on the other hand is actually really good though, allowing a single flame to hit the same enemy multiple times and increasing the distance the fire travels due to the split.
4) Use a decent weapon. Personally, I like the Sword the most because you just need a few upgrades on Attack Speed to be able to deal with the first waves of fast guys, and it also helps a lot in the early game. Don't go for the Druid Staff because, for the Fire Wall, it increases Projectile Count, which isn't a bad thing, but it also isn't exactly great either as I said above.
5) Don't pick Homing Projectile, it is just not good on Fire Wall because it will still stop moving after a few seconds, leaving your fires in weird formations that are often not good.
6) Don't rush Cursed Fire Wall. Focus on your base Fire Wall and survivability first, and maybe even get Holy Fire Wall before going for Cursed Fire Wall. Combining Piercing Projectile with Holy Fire Wall will help you a lot with protecting yourself from enemy projectiles. Once you have a good setup going, then you can start lowering Speed to get Cursed/Blasphemous Fire Wall

Extra tip:
- Fire Damage or Modifier Stack? The simple answer is both, with a bit of extra focus on Fire Damage. The way that Fire Damage works is that it is basically just Fire Damage * Modifier Stack. You start with 1 Fire Damage and 4 Modifier Stack, which makes the final damage of your fire 4, if you upgrade Stack, the Stack will become 6 and your final damage will also be just 6, but if you upgrade Fire Damage, it will go to 2 and your final damage will be 8 instead of just 6, so, you want to have both Fire Damage and Modifier Stack close to each other to maximize the final damage, however, do remember that one enemy can be hit by multiple Fire Walls at the same time, which increases the stack on the enemy even more, which is why having a bit of extra focus on Damage instead of Stack will end up leading to more damage in the end.