The Sunburst effect is not a built-in filter in Fireworks, you will have to use a third party filter compatible with Fireworks. So, first of all, download the free and fireworks compatible DBS filters with 8BF extension at the bottom of this page. Drop them in your plugin folder Fireworks (whatever your version) > Plug-Ins then restart Fireworks. From the 21 filters we will just use the Mirror Center filter
The Gradient Fill
Select Rectangle Tool and draw a rectangle then fill it with a linear gradient going from white to black.
Figure 1: Completed Tutorial
Move the black color selector to the center then the white on top of the black selector to achieve a black and white rectangle.
Figure 2: Completed Tutorial
Tip: Once your gradient is made, save it as a Style to be re-used more easily throughout your documents. With the rectangle still selected, go to Assets panel > Styles tab > New Style
Mirror Center filter
With your rectangle still selected, go to Filter menu > DSB flux > Center Mirror. Select 15 rays and no value for rotation but you may choose your own values.
Figure 3: Completed Tutorial
The result is a bitmap image with black and white colors.
Figure 4: Completed Tutorial
To keep the sunburst rays we will apply a Convert to Alpha filter by selecting Filters > Other > Convert to Alpha. The Convert to Alpha filter will delete the white color and replace it with a transparent color and will keep the black color.
Figure 5: Completed Tutorial
Modifying the colors
You may modify the colors of your linear gradient to obtain a bi-color sunburst effect (see step 2). Here is our example: #0061A0 -> #079DFF .
Figure 6: Completed Tutorial
Alternatively, you may Convert the bitmap into a path. For the Fireworks 8 user only. After the convert to Alpha filter step, is a good alternative option to:
ALT+click in the sunburst layer to get a selection
Go to Select > Convert marquee to path. (antialias)
With these easy 2 steps you get the path in vectors so you can resize it or modify its colors or whatever.