Attached Link: Stefan's Firefly FAQ

Everything you wanted to know about  Firefly but were afraid to ask. 

There's a bit of a learning curve with Firefly.  It's not difficult, but it does require more thought than the P4 renderer.  The best settings will be different for each render.

Use the manual settings, not the automatic.  For the problem of jagged edges, try decreasing the shading rate and/or increasing pixel sampling.