Thank you for your post.
I have FileMaker Pro 9.0v3 Advanced running on both Windows XP (sp3) and Mac OS X 10.5.6. I have opened files on both machines and turned on Instant Web Publishing. I can view the files using Firefox 3.0.10 on Windows XP and Firefox 184.108.40.206 on Mac OS X 10.5.6. I know this doesn't help you, so I'd like to find out more about your environment.
1. Does this happen with all database files or just the one?
2. What version of Firefox are you using?
3. What computer (including OS version) is the hosted file, and what computer (including OS version) is using the browser?
4. If you have access to another machine with a browser, does it also have the same problem?
Any additional information you can provide may be helpful in determining possible causes.