Packet forwarding on OS X Mavericks
The Internet Sharing service will handle all the translation requirements by itself. For some reason, though, it's necessary to use the Network Preferences pane to manually confirm the information the bridging interface has already come up with on its own. (Since the BBB has its own DHCP server, perhaps the Mac is willing to accept the BBB's DHCP offer in the 192.168.7.x subnet as a suggestion, but then reinforces the fact that as the DNS it holds the keys to the external world, thereby imposing its configuration on the communication process. I don't know. I do digress, however.) 

After turning on Internet Sharing for the BBB in the Sharing pane, switch to the Network pane of System Preferences. Select the en6 service in the lefthand window, click "Advanced" at the lower right, and select the "TCP/IP" pane. If it's not already filled in, Under "Configure IPv4," select "Manually" from the drop-down menu, enter 192.168.7.1 for both the IPv4 and router addresses and 255.255.255.252 as the subnet mask. This leaves 192.168.7.2 for the BBB and 192.168.7.3 as the broadcast address. You can ignore the IPv6 configuration section. Next, select the "DNS" pane from the top row of tabs. Click on the '+' mark just below the window and add 192.168.7.1. In the right hand window, add "local" (without the quote marks) to the search domains.

FYI, I'm sure you'll want to know that ipfw has been deprecated and replaced by pfctl in OS 10.9.