Fireworks is a graphics editor that can export code; it is not an HTML/code editor.
If you have exported HTML from Fireworks, you ought to be able to re-import that, but I would think most people who use Fireworks exported code would just modify their Fireworks document and re-export. (Fireworks' code is not robust and it is only intended for mockups and prototypes. Fireworks is not intended for developing and maintaining live Web sites; Dreamweaver is.)
If your developer used Dreamweaver, then I really don't expect that Fireworks would be able to import that, nor any code that isn't in exactly the format that it can export. If you know your developer used Fireworks, then you need to look for .png files. Fireworks uses an enhanced version of the PNG format for its native document format.
If you are making small adjustments to your site, then you might be better off working in Dreamweaver. If you're doing a site re-design, then Fireworks is an excellent tool for laying out your design and producing your graphics, but you will, at some point, need to move over to Dreamweaver for coding.
Thanx for the quick response Pixlor. Maybe I should just take some screen shots of pages in our current site and then recreate them in Fireworks and then re-arrange them, enhance them, try new layouts, etc.I find Fireworks so much easier to use for design than Nightmare Weaver.
If you want to experiment with different layouts, then starting with some screen caps would be a good idea. You can import any existing images, though, so you don't have to slice or crop those out of screen caps. Fireworks and Dreamweaver are complementary: Fireworks is great for the layout and design work, Dreamweaver is great for coding, once you have your design ideas more or less set.
If you come up against anything that you have questions about, post back, though. Very few questions here go unanswered.