First, put 'door on fire' and 'front door' in the same room.
Make 'front door' unavailable (under the advanced tab of the object).
Create the verb 'step on' for the object 'tile', or make the command 'step on #object#, then run script as follows:
Print message "As you step on the tile, the door is no longer on fire, granting you passage further into the dungeon...", make 'door on fire' unavailable, make 'front door' available (accessible) and unlock the exit (remember, from "dungeon room 1 to direction south", not from "dungeon room 1 to dungeon room 2", otherwise it won't work).
Hope this helps.