slowdown particle movement

is there a way to slowdown the movement of particles?

because i want to create a simple particle around the tree that will look like flying firefly

You want an Ellipsoid Particle Emitter, as well as a Particle Animator and a Particle Renderer. Change the properties of your particle emitter. World Velocity and Local Velocity are what you are looking for. You would probably want a script attached to the same object that changes the velocity variables slightly so that not all of your fireflies fly in the same way, or stay exactly still. Or having zero velocity could work, so you would just have a lot of blinking lights around your tree.