I remember reading these books a long time ago.
In each book a new alien race is discovered.
Each alien race is associated with a classical element: fire, water, etc.
They fight each other while Humans (or a humanoid race) is stuck trying to deal with the fallout.
There were about 5 books in the series.
I think the "fire-type" aliens where called the Furios(?) or something similar if that helps.