Firmino hasn't been out for months, years, decades etc, where as Sturridge has been.
Sure, there is a difference, but Firmino has not been training until maybe the past day or two? Klopp wasn't even clear he would be fit enough for the game which usually means on the bench at best. Suddenly, he's good to start.
Klopp says: "After coming from injury, if you could immediately play 90 minutes that would be perfect. But, unfortunately in football, the real match is too intense that you can work that hard in training."