In reply to Twiggy Diablo:
I used them in Alicante in December 2019, primarily because they were so cheap and because I knew I had a decent enough 3rd party excess cover and booked through a third party (rentalcars) who I knew sometimes step in to help resolve issues.
I was a bit worried given the reviews, but it was basically okay. On reflection, unless I was absolutely pinching the pennies I wouldn't use them again.
At the desk in the airport the clerk had to be asked several times not to include the various forms of additional cover from firefly ("oh, your excess cover also covers tyres? Okay I'll remove it").
Picking up the car i checked out over fully and it had lots of unmarked damage, which I documented and had added to the paperwork. Dropping it off there was a bit of a queue (though we arrived early knowing this would be an issue) and they checked very thoroughly for damage but only found the things I'd had added.
On the plus side I got a week's car hire for just over £30 (if I remember correctly). On the negative side there was quite a bit of haggling and checking to make sure that was all we paid, and I'd pay a bit more just to skip that next time