Hi there!
I'm renting a minivan for a week using Firefly because they had an amazing rate of 270 dollars (tax included), while other companies were around 600 to 800 :-O
I never knew the company before, but since it came up in a comparison through orbitz.com, I imagined it was trustworthy.
So I did the reservation and got an e-mail confirming it that was <webmaster@hertz.com>, and then found this forum with some people mentioning that Firefly was acquired by Hertz.
I called the number provided in the email (8007775500) and it was from Advantage, if I'm not mistaken, and they said they've been getting the same question lately.
Then I called Hertz at 800-654-3131 and they confirmed the reservation just fine, and even sent me a new email.
This time the confirmation email came as: "Firefly Reservations <fireflyreservations@fire.flycarrental.com> via hertz.com" and the phone number 1-888-296-9135 for reservations, and also an additional number which seems to be for the local office at Orlando Airport (4078598733).
The 1-888 number picked up and seems to be the same thing as Hertz, but says its from Firefly instead. The local number at the airport didn't pick up though, but all in all it looks like Firefly is for real and I guess Hertz is still working on integrating it into its existing processes and customer support.
Hope all this info helps!