Firewalls prevent unauthorised data being transferred from and to your computer. This effects data between applications connecting websites. Usually firewall users can "allow" permitted applications to transfer data and set various levels of security.
For example, XOOPS sites use the HTTP_REFERRER information to check if you're logged into the website. Some firewalls (most notably software firewalls such as Norton and ZoneAlarm) block this information being sent to the server. This could prevent you from logging into the XOOPS site. Typically this becomes evident when you log into the website, the login is confirmed, and you still see the 'login' box on the page, and you're not logged in at all.
The solution is to add the XOOPS site you're trying to log into to the Trusted Sites list of your firewall application.