The router's firewall has nothing to do with this issue. That's only effective on WAN->LAN traffic, it doesn't affect local traffic on the LAN side.
Other than preventing a connection at all with MAC filtering, the router doesn't have much say over local networking.
The master browser election is being screwed up, probably by firewalls or Internet security applications.
Since you are behind a router, the first thing I'd do as a test is disable ALL firewalls on ALL machines. Get it working that way, then look into firewall configurations.
FWIW, the only firewalls here are the router and if I forget to turn off the Windows firewall on a machine.