This link says it better than I can:
Basically, the Web Layer is the top layer in a document and cannot be moved from its position in the layer stack. It contains slices and hotspots (if you've created them). It doesn't contain text, bitmaps, or vector objects.
The vector shapes I originally created were not slices, nor hotspots.
I had created an A4-sized .ai file in Illustrator, opened it in Fireworks, and saved it as a .png.
I then copied my vector shapes from the original png, and pasted into the new png file. This is where the problem started. Why have my vector shapes become associated with the Web Layer?
I don't know... Have the shapes been converted to slices or hotspots (with a green, translucent appearance)? Can you try pasting the vector shapes again, from the original PNG file?
Pasting the vector shapes again worked. Thanks groove25.
The original pasted shapes were definitely not slices or hotspots - they looked just like they were in the other document I copied them from. Weird.