Firemouths are one of the more difficult CA to determine sex. In a group, sometimes it can be obvious as you are dealing with fish at the same age that can be compared.
Based on body shape, it appears to be female. Certainly an older male would have a different body shape.....but even a young male could have more of a female body shape then an old female.
Just about any cichlid can be reliably sexed by observing the breeding tube over an extended period of time. Based on your picture, the tube is thick at the base, and at least in that picture from that angle, appears to me to be more female. If it is female it will change a lot over time as the female organ is much more in flux. It goes from not protruding at all to very lafge, thick and blunt just before laying eggs. A mature male will usually have something protruding at all times. It is pointed at the tip and is thinner then the female organ. It does get larger shortly before fertilizing eggs but is always long, skinny and pointed.