I have a similar situation.  What I did is create a "stub" VLAN with only the firewall/router on it, and one port of the 6248 switch on it.  For example, set up VLAN 7 on the 6248.  Give it ip address 192.168.7.1.  Assign one port on the 6248 to VLAN 7 and enable routing.  Connect this port to the firewall.  Set the LAN interface of the firewall to 192.168.7.2.  Set the default route of the 6248 switch to 192.168.7.2.   This should allow all devices on your other VLANs to reach the internet.
Hello everyone, I searched the forums but haven't found a clear anwser to this.  We have a PC6248 with 3 vlans: servers, workstations office and workstations plant. The servers vlan are on 192.168.0.0/24 with default gw 192.168.0.1 (pc vlan10 ip address) On PC 6248 I've added ip route default 192.168.0.2 wich is the internal interface gateway ip.  Internet and routing between vlan is working, however there area high latency an timeouts between vlans during the day that ocurrs for about 10-40seconds and they repeat about 10 or more times a day.  I talked DELL techsupport and the gay says the problem is the default route and it must be removed  but I told him I cannot remove it cuz no one will have internet access. I attach picture to try ilustrate better and will appreciate any help about this.[IMAGE] 
Howto make Powerconnect 6248 work with a internet firewall/gateway.