I just went to the create an account page and this is what it says.
"The administrator is currently not accepting new membership registrations. "
I found a more recent post from the administrator about the problem. Apparently, FPN has been the target for a lot of robot spammers, and to protect themselves, they have had to resort to manual validation - the automatic schemes such as CAPTCHA aren't very effective. The administrator noted that his day job involves traveling most of the week, so he can only work on the site on weekends. As a result, they have a problem validating new membership registrations.
You can lurk on the site without registration. As a workaround, I would be happy to post questions for IAP members who are unable to post questions on FPN.
By the way - I've noticed that Firefox has evolved in a way that is presenting a number of problems. One of the most troublesome is that it does not always display the CAPTCHA log-in validation app. For the last several months, if I have had a need to access a site with CAPTCHA verification, I have had to use a different browser. That probably doesn't explain the problem at FPN because they appear to have concluded that CAPTCHA is not reliable, but it could be a problem on other sites.