Generally speaking, you're better off finding a display typeface that already has the bold appearance that you're looking for, or one that includes heavier weights such as Bold, Heavy, Black, or Extra Bold.
The bold style in Fireworks is an artificial bolding that's added on to the typeface. If it's not enough, you could experiment with adding a text outline, but this usually creates a pretty muddy appearance. (Text outline is in the Properties inspector, next to the aliasing options). If you're really thickening up the text, you might want to also loosen the tracking or kerning (also in the Properties inspector).
You could try this: In properties panel (see right under Bold | Italics | Underline buttons) 1) Change Anti-Alias Level to Strong Anti-Alias 2) Check Auto Kern check-box
If this doesn't help you could duplicate the text: Ctrl+C Ctrl+V for a quick fix to the problem.