It's an extremely powerful spell, especially with enhancements or cast with empower and/or maximize.
It makes some things completely trivial it's that imbalancing, the AI completely ignores the danger, and players get free run to run across their own firewalls or even fight in the middle of them untouched.
Also the spell makes the same shape all the time, it's a rectangular piece.. which is often cast in midair by the game interface.
Suggestion 1: Make the firewall made up of about 6 pieces of fire.. each of which falls down onto the surface separately so you cannot make a firewall in midair anymore.
Suggestion 2: Make the spell deal 2 fire damage a click to even friendlies standing inside it.. but no damage if you're near it.. so a lesser fire resistance item will keep you immune or a resist fire. This should also include monsters standing in their own firewalls.. subject to the same rules.
Suggestion 3: Make the spell MODAL with 2x options.. the second options being a ring of fire around the caster themselves. (the 6 pieces of firewall from suggestion 2, just get placed in a different spot)
Suggestion 4: Make the AI take notice of dangerous persistant spells like this and at least the intelligent monsters (look at their stat) should try and path around or at least out of the dangerous areas.