Each only takes 1 NCU and last 3 minutes. You will also have a glowing ball over your head, indicating you have some enhanced abilities.
Shadow Rifts are the same QL as mission they are in, so while at low levels the gain might be minimal, they give quite a big boost at higher levels.
Since they can be found in team and solo missions, teamed players must decide who will use it (they can only affect one player then the rift vanishes from view).
Note: Firefly is usually used by tanks (mongoing enforcers) or by healers. But it's really the players decision, who will benefit most from it (or it should be, teams are as strong as it's weakest member).