**READ ON IF YOU ARE LOOKING AT FIREFLY BECAUSE OF THE CHEAP RATE**
I strongly believe it's just their marketing scheme to bring customers in.
They do NOT have the online rate at the counter. The $11-12/day rate is for online reservation ONLY and some have said even after reserving they were still over charged afterwards. So unless you have 2.5hrs to wait for your res, you won't get that rate.
Now, even if you do get that rate, it will come up to the same as Enterprise with their other bs fees. Firefly adds a whopping $25+ per day insurance fee without your consent. Even after I inquired and was told it was OPTIONAL I was still forced into buying it. After that you'd be asked if you want to pre-purchase a tank of fuel. They claimed its 40c cheaper than market. I initially thought it sounded pretty good so I purchased a $26 tank. Later I came to realize, with Mitsubishi Mirage's tiny 10gal tank, it cost me roughly $22-$24 to fuel a COMPLETELY empty tank AT MARKET PRICE. ($20 was enough to go from one bar to full) It's evidently a trick they play to get more $ since most drivers won't and can't return the vehicle with a "complete" empty tank. To play their game I made sure to return it when the car estimated 20mi left in the tank. (no bars on the meter, sucker)
At the end, I spent ~$140 to rent a Mitsubishi Mirage for 2 days. Yeah. In the past I've spent just a pinch more at Enterprise for a Yaris for 5 days. The vehicle was new though still crap. TINY, no cruise control, bad blind spots, non-existent trunk and LOUD! The only thing I give props to Mirage is its BEAST gas mileage. Driving to LA from Vegas with a 20buck tank, cruising (manually) at ~75-80mph with a 40+mpg average. (Maxed at 47mpg, yes, beast)
In short, just like many others have suggested stay away if you could. If it's too good to be true, it is.