So what happened with the magnet link? You said it started to downld. Did it stop or did you cancel it?

My guess is if you're getting a firewall with FW but not with LW, then they're both trying to access the same port in the same session. Change the listening port of one or the other. And even the connection port if possible. ie: temporarily turn UPnP off, change connection port & listening port to a high number. Then turn UPnP back on. I'm not sure if this will work but just a thought as an experiment. Since Upnp will tend to choose its own ports sometimes. You may need to restart the program for it to work. I'd suggest you do this for FrostWire since that's the one that's seeing the firewall.