When alchemical fire is on the ground and an enemy hero is affected by Mirage's tornado they will not take the tick damage from the alch fire but they will have the weapon damage amplification applied.
If you buy extended duration and apply it to tornado then the alch fire will start to tick only during the extra duration added by ED
If you buy extended duration and apply it to tornado then the alch fire will start to tick only during the extra duration added by ED