There should not be more than one software firewall running on a system as there is a very good chance they will conflict with each other and have seen this happen before. On the other hand, if the user has a hardware firewall (such as in their router) and a software firewall, there generally is no risk of both causing problems with each other, however, what rules are set up on one must also be set up on the other. Generally most router firewalls outperform the software firewalls what it comes to blocking incoming attacks, with the exception of on any ports the user specifically opened. The only main thing hardware firewalls don't do (which most software firewalls do) is warn if a new application is trying to access the Internet, but generally a good virus checker would catch most unwanted malicious software, viruses or worms before it even gets a chance to run and start accessing the Internet.

Then again, I cannot see firwalls causing the issue they are experiencing, unless they all installed or updated their firewalls around the time they first started noticing problems with Limewire.