From the Super Mario Wiki
A Flower Jar increases Mario's maximum Flower Points by three in Super Mario RPG: Legend of the Seven Stars. Flower Jars are hard to find, and are often received after defeating boss characters. Additionally, Flower Jars recover all of Mario's FP.
Note: Flower Jars need to be used on the item screen before Mario's maximum FP increases, unlike Flowers, which automatically increases Mario's maximum FP as soon as he picks them up.