Hi,
concerning firewalls, of course your built in mac FW may be properly configured to let LimeWire get through (by the way, normally, I believe LW can fiddle its way through a FireWall, by him self and like a big boy), but sometime, they are other stops one doesn't think of.
Of course, Net Barrier is a proper FireWall, so as Peerguardian, but aren't you running by any chance "LittleSnitch", which prevent any outgoing communication (for example from any app "phoning home")...
and also some time, an Antivirus, I know we don't use many of them regularly on our macs, and mostly only for the sake of friends running windows, but that one reason that some time we forgot about them, and when they run, they act as a sort of firewall... Of course, depending of your ISP (not talking of the ones filtering knowingly the P2P... I had one like that [free.fr] that I booted out, telling him why!), you may also have the ISP firewall and, some time the ISP's router to configure.
I'm now with "Orange", and the modem ("LiveBox") is also a router. I removed on start it's firewall, and then still had some fun to connect my clients (LimeWire, eDonkey and Ants), before I realised that there was a router with the "LiveBox"... If that's the case, you only need to enter the port your are using with LimeWire (it is the same one for TCP and UDP) by the way I've read somewhere that it is better not to use the default port of Limewire, and to change it in the prefs, so I'm using 46169...
Good luck