Playing the game as it's currently available on Steam (version 0.18.1722 I believe) with no mods. Have found that Firefoam shells cannot be loaded into Mortars.
I built a shelf next to a Plasteel Mortar and loaded the shelf with Firefoam shells. If i tell a colonist to man the Mortar, the game says there were "no nearby unreserved shells" for it.
I built a shelf next to a Plasteel Mortar and loaded the shelf with Firefoam shells. If i tell a colonist to man the Mortar, the game says there were "no nearby unreserved shells" for it.