The best source for an answer to your question is Firefox itself. Only they know their intimate specs such as the grade of steel and allow hardness and under paint dimensions. The steel used for my MTB hangers is mild and 4mm thick and has finally opened up from the torque produce by the HBS-36 motor. I'm going overboard on the fix with a 10mm closed hole torque arm where a 6mm closed hole might do. I would also avoid alloy forks altogether, even with a torque arm long enough to reach the headset.