Not clear what you mean by data flow. Do you mean data going through SW1 or SW1 talking to the firewall itself.
Either way, if the mac-address for the firewall interface is in the mac-address table on SW1 and the data is coming from the same vlan then the traffic will be sent directly from SW1 to the firewall. If the data is in a different vlan then it would need to be routed onto the correct vlan then sent to the firewall.
On a more general note having a switch as a root switch for a vlan does not mean all traffic for that vlan must go via the root bridge. STP elects a root bridge as central point and then works out a loop free topology within the network. But that doesn't mean that all traffic must pass through it.
Jon
