You can open and edit several types of image files with Fireworks but if you want to save your edited image as a .gif or a .jpg you will need to use the "Export" command and not the "Save" or "Save As" selections. The default saved format is .png, especially when using the "Save" or "Save As" selections.

When you are done editing the image go to <File>,<Export...> and then select where you want to save the file and make sure it is using the extension you desire.

Simple once you know it but confusing for the beginner.