Creating a Bitmap Mask
You can create a bitmap mask by selecting an area of the document through which the bottom objects will show. The bottom that will show through will be the document canvas color.
Open a document in Fireworks. You can use a document that contains vector objects, bitmap objects, or a combination of both. I am using a picture of my dog Cracker. If you would like to use this for this tutorial just copy and paste it to your folder:
Choose Modify > Canvas > Canvas Color. Click the custom radio button then choose a color from the Color dialog box by picking a color in the photo using the eyedropper tool then click OK.
Click on the marquee tool on the tool panel and choose Oval Marquee:
Set the Edge tool settings on the properties panel as desired:
For Marquee tool selections only, choose a Style on the properties panel:
Draw a selection marquee around the area of the document that you want to show through the mask.
Click the Add Mask button on the Layers panel. If the Layers panel is not already open, choose Window/Layers to open it. Make sure the bitmap is selected. You can tell by looking at the layers panel and it will appear blue if it is selected.
The color you selected as your background now appears outside the marquee area.