Well I guess I should chime in here, so to speak. I installed Fireflys in my A54 a few years ago. They lasted about 3 years before the voltage drop started to cause problems for me. They were promoted as being "almost" as good as Lithiums for about half the price, so I thought why not.
I cannot say why they failed, but as we were yanking them out of the boat, we load tested all of them just to see how they had held up, thinking maybe 1 or 2 had simply died. Almost all of them showed significant degradation. I was able to salvage a couple of them to serve as starter batteries, but I was not a happy camper. I do have a very heavy electrical load. Perhaps the 12-volt in series configuration is not compatible unless you deliberately balance them out periodically. With 24-volt Lithiums, that is no longer a concern.
Despite the expense (2 times the Firefly technology), I am convinced that LiFePO4 is the way to go if you are looking for long-term serviceability. They hold their voltage levels even with extended bow thruster use, and I never get low-voltage alarms on my Maretron N2K bus any more. I used to get them all the time after the first year and had to wake up in the mornings to start the generator to quell the alarm. I had the alarm set to go off if the voltage dropped below 24 volts.
Sorry to put a bummer into the carbon-foam technology, as some may have had a good experience given the "better mousetrap approach," but they still are based on lead construction. I expect in the future LiFePO4 or some derivative will become the norm for our Amels.
Phantom, A54 #44