If this is the case, then you will be doing a double-NAT configuration. In this scenario, the network segment between the WAN interface of your pfSense box and the ethernet interface of your 2811 should be treated as your WAN network segment. The down side to this type of configuration is that you cannot allow any inbound services (SMTP, HTTP, etc) without having your provider create port forwards for you on the 2811 and then you repeat the same configuration on your pfSense box. Your network would then look something like this:
Office LAN -> pfSense LAN -> [pfSense box] -> pfSense WAN -> 2811 Ethernet -> [2811 box] -> 2811 Fiber connection -> Internet