A fire is achieved when a flammable vapor reaches a hot element that is above the flashpoint of the vapor.
In this case, you heat the liquid until vapor begins to fill the container. The heavy vapor will rise over the edges of the container and drop downwards due to the density compared with the atmosphere, and will ignite when it hits a surface heated at or above the flashpoint.
This can take place over distances of 6 feet or more. I have seen it, and it is much more difficult to contain and extinguish. The sudden ignition of even a moderate amount of vapor can be nearly explosive in force.
Very often, this is seen with gasoline or solvent based paints which can ignite when even opened in a shop with a lit pilot lamp on a water heater or furnace over 6 feet away. I was a consultant for an attorney on such a case many years ago.
In many cases the ratio of air to vapor determines whether you get a fire or an explosion.
IIRC, TF-3 uses Borax for buffer. I don't have my copy of A&T handy, but if so then the Borax would have to be essentially free of Borax. Common salts vary in the Potassium and Sodium ratio around the world so it could depend on the source.