If you include players like Salah and Sterling as forwards, then I would agree, but if the list is limited to center forwards, then he's certainly top 5. I'd have Kane, Aguero, and PEA ahead of him, but that's it. And even then, I'm not sure I'd trade Firmino for any of them. His unselfishness helps the offense flow, while Kane in particular would clash with Salah. Firmino is also so durable.
As far as Liverpool's squad goes, I think Salah and Mane are better individually, but if I can only have 2 of the 3 available, I'm taking Firmino and one of the other two. Their talents overlap with each other's in a way that his dont.