I get the "Fonts not available" message when I open up a document, i.e. open up a png document in the program/computer/etc. where the program was created in the first place.
I don't get this. How can the fonts not be available when I'm using the same program that used these fonts originally? Having said that, the appearance doesn't seem to change.
Are the fonts in question installed on your computer?
Fonts are not part of a program, fonts are installed on a system. Once installed, a font is available to any program that uses fonts. If you're opening a document that was made in the same program, but on a different system, using fonts you do not have installed, then you will get that message.
So... Are you opening a document that you created? Are you opening the document on the same system on which the document was created? Have you made any changes to the installed fonts (installed or uninstalled any fonts)?
What version of Fireworks are you using and what OS are you running it on?