How to remove background from png in fireworks?

Really depends on how intricate the background is, a plain colour or large area of simiar colour might be doable with the magic wand tool and adusting the tolerance, you may want to adjust the edge setting too. For intricate background removal you are better off with a tool like Photoshop which has more advanced tools for this type of task.

Select your background with the Magic Wand tool.  See this video for details.