If you have Salah and Mane either side, you can get away with Firmino.
If he played in our side, i think he'd be fairly dreadful.
But in terms of this season it's an utterly no brainer. Martial coming off by far his best season, even missing 6 weeks or so, versus Firmino who barely scored at home all season.
These type threads often don't age well though.
I'm sure there was a Martial v Abraham one at one stage, and who could forget the Shaw v Danny Rose one, where about 95% of people were adamant we should go for Rose (me included to be honest)