"To make an image transparent, I usually use the Fill tool with the Erase mode"
That seems like an unusual work flow to me. If you want an entire image transparent, why not just hide it? If you want it partially transparent, why not use Auto Vector mask? What effect are you trying to produce?
I'm talking about taking an existing non-transparent image and getting rid of the solid background
The alternative I suppose is to use the magic wand tool, but that doesn't fix the application bug.
I've never run across anyone trying to make an image transparent in that way. Here are some tutorials on the traditional ways of selecting or masking for transparency in Fireworks.I don't think what you are describing is a bug.