Silly question what is the differences between Grimdark Future and Grimdark Firefight? To me it seems they both use the same stats.
The difference is scale. Nearly all units in Firefight are cheaper, but come in singles. For instance, a Battle Brother  is 20 points in Firefight, and Battle Brother  in Future is 110. Battle Sister  is 15, and Battle Sister  is 80.
The idea is that it's a much smaller engagement. Instead of 500 or 1000pts, you have 150 or 300Pts. Instead of 20+ models among 4+ Units, it's just 4-7 models each taking their own turns and fighting.