Thank you for your post.
I am unable to replicate the problem. This is what I have done...
With FileMaker Server 11 on a Mac Server running OS X 10.6, I use the FMServer_Sample.fp7 file. On other machines (iMac running 10.5.8, Mac Mini running 10.6.4, MacBook Pro running 10.6.4), I launch Firefox. Under Preferences, I disable cookies. I then enter the IP Address in the browser and the Instant Web Publishing page displays showing FMServer_Sample.fp7. I am able to select it, make changes, and log out. I did this several times on each machine without incident.
Does this only happen with the one file? Or, can you try this with just the sample file and see if you can replicate the issue. What version of Firefox are you using? Are you running any scripts in the file? Although your description is excellent, can you provide a little more detail to help me try and replicate the problem?
Please keep me updated with any progress.