Here the vendor offers their product at a heavily discounted price to start off and then increases the price after x number of units are sold.
Another form of fire sale is the wacky Wednesday girls using the dime sale tactic. Very effective for creating mad cash and a list of KNOWN buyers.
Another well known tactic is and has been used by Jeremy Burns with his sourcecode blowouts sales over a 7day period.
Then there is MR Frank Kerns 4 day 50% off sale. I do class this as a fire sale because you are offering a discount to entice people to buy; that may have been sitting on the fence.
So what type of fire sale do you like or dislike and if you do run them; do you automate the task of changing prices, dates, quantities and text or do you have some means of doing this on auto-pilot.