what the other comments said, but also know that (iirc potions of cleansing or purity) any source of water (aqua blast, geyser traps) or frost (blizzard pots, potions already mentioned) should do the trick against the fire wall.

1 thing to note is that every puzzle type room force-spawns a specific solution to it in the same floor's loot, outside of it's own rewards (for inatance this room forces a potion of frost to be somewhere on the level, or a chasm/trap-filled room forces a potion of levitation)

edit: by the way, icecap plants work. the issue is that it's slightly tricky to do so, as if you throw any plant against the wall and the plant lands on a non-water tile, it burns in the next unit of time (not even a turn, near-instantly) so you have 2 ways to easily do it: 1 - plant it on an adjacent water tile to the fire if there is one, then the plant won't burn (it only burns if fire spreads to the terrain it's on as all plants do, and it's very specifically not on a burnable tile) then trigger it to clear the fire 2 - use a swifthistle first, then throw the icecap seed next to the fire and immediately throw something at it (that won't freeze over, unless it's raw meat, of course) to make the icecap trigger the moment time stop ends and clear the fire (also works with hourglass timestop)