You'd personally would have to keep the Fire there with consistent Firebending on your part for this work. However, this also means other Firebenders would be able to bend the fire away if they wished (And you could technically counterbend and etc).
So, I think the ability would be more suitable if worded such as, "Can withstand being used as an extension for firebending without damaging the sword itself".
You would also need to reach the level where you could sustain that type of firebending around the sword. Otherwise, the only way to have you just touch it and keep the fire there is to say dump oil on the weapon.
Also, keep in mind despite there being no restrictions you do not need a crazy amount of weapons. Just fair warning because that's what caused restrictions on other sites I've been on.