Hi Ninjai,
A vlan on a juniper switch is its own boadcast domain. It groups all devices into a specific (vlan) group as per the vlan membership of the interface on the switch through which the devices are connected.
Whenever there is a need to split a interface into multiple sub interfaces then the unit # is referenced. A unit number identifies the particular subinterface of the main interface. JunOS supports 4094 logical subinterfaces per main interface. As a general practise it is recomended to use vlan id as the unit number for better clarity and visibility of the configuration/network.
In your case you can use irb to configure ip address to the vlans on the switch. However the switch does routing for the packets which are have gateway/nexhop as the vlan ip address on the switch. You can still point to the Firewall ip address connected to switch port 23 as the gateway for the hosts in your network so that your routing is performed by the firewall.
Following is an example vlan configuration.
set vlans vlan100 vlan-id 100
set vlans vlan100 l3-interface irb.100
set interfaces irb unit 100 family inet address 172.16.4.3/24
set interfaces ge-0/0/5 unit 0 family ethernet-switching vlan members vlan100
set interfaces ge-0/0/23 unit 0 family ethernet-switching interface-mode trunk
set interfaces ge-0/0/23 unit 0 family ethernet-switching vlan members vlan100
In the above configuration the traffic from port ge-0/0/5 will be switched to interface ge-0/0/23 as long as the default gateway for the device connected on port ge-0/0/5 is set to Firewall ip address connected to port ge-0/0/23.
Please refer to the following documentation for further details.