Thank you for your post.
The first version to be certified with Mac OS X 10.5.x was FileMaker Pro 8.5v2. FileMaker did not spend time with FileMaker Pro 8 and earlier versions to run on Mac OS X 10.5.x.
I was able to install a copy of FileMaker Pro 7 on my Mac OS X 10.5.7 machine, but I cannot turn on Instant Web Publishing, so I am unable to duplicate your problem.
The current version is FileMaker Pro 10. You can download a trial version from our web site and try it out. Turn on port 591, shut down and restart. You will see that port 591 is saved.