Ok, your modem does have a built-in firewall. This might be the cause of your probs. I'm not sure about port forwarding through it as the portforward site doesn't have your modem on it's list.

But...it does use UPnP. So, if your UPnP is enabled within the modem you'll have to enable it in LW too. Go tools > options > advanced > firewall config & tick 'use UPnP'. Now click on apply.

Do this test:
http://www3.limewire.com:6346/

Does it work?