I think the original Firefly operational notes thread is now too long for any new info to get read so: It seems the recovery procedure with it's 0.4C charging rate is causing a lot of confusion. So I
One of your challenges will be to see if they fit our standard battery box in front of the galley.
The other will be to assure that your charging system is up to the task of charging them as required, although your AGMs may have resolved that issue if you've upgraded your charging system to meet their requirements.
Much also depends on how you use your boat: anchor out or marina hopper.
If you are the latter, I can't see even AGMs not to say FFs as a necessity.
Your boat, your choice.
There are also many Firefly hits on our C34 forum, one really good one by Maine Sail: Main Message Board (Do a search on Firefly in the search box, it will be the second one down, search results never transfer in links).