I started working on Juniper equipment around 2002. At my employer, we had an M40 with the serial number 256.  We did Layer3 only.  I had no idea if the Juniper even did layer2.  It certainly wasn’t a layer3 switch like a 6500 like I was used to.  It was like a deliciously robust version of any Layer 3 router I’d worked on previously.  Over the years Juniper has added a switching line utilizing their FreeBSD based OS, JunOS.
All that being said, I’d never really messed with doing a layer2 transit VLAN on a JunOS routing platform.  Lets say we want to make 2 VLANs and transit them up through the WAN.  Here is how to make it work on an MX:
Create the bridge domains:

set bridge-domains vlan-123 domain-type bridge
set bridge-domains vlan-123 vlan-id 123
set bridge-domains vlan-124 domain-type bridge
set bridge-domains vlan-123 vlan-id 124

Configure the interfaces facing south (LAN) to nbe members of the VLAN:

set interfaces xe-1/0/0 description "SW1 xe-1/1/0"
set interfaces xe-1/0/0 mtu 9192
set interfaces xe-1/0/0 unit 0 description "Untagged VLAN 123"
set interfaces xe-1/0/0 unit 0 family bridge interface-mode access
set interfaces xe-1/0/0 unit 0 family bridge vlan-id 123
set interfaces xe-1/0/1 description "SW1 xe-1/1/1"
set interfaces xe-1/0/1 mtu 9192
set interfaces xe-1/0/1 unit 0 description "Untagged VLAN 123"
set interfaces xe-1/0/1 unit 0 family bridge interface-mode access
set interfaces xe-1/0/1 unit 0 family bridge vlan-id 123
set interfaces xe-1/0/2 description "SW2 xe-1/1/2"
set interfaces xe-1/0/2 mtu 9192
set interfaces xe-1/0/2 unit 0 description "Untagged VLAN 124"
set interfaces xe-1/0/2 unit 0 family bridge interface-mode access
set interfaces xe-1/0/2 unit 0 family bridge vlan-id 124
set interfaces xe-1/0/3 description "SW2 xe-1/1/3"
set interfaces xe-1/0/3 mtu 9192
set interfaces xe-1/0/3 unit 0 description "Untagged VLAN 124"
set interfaces xe-1/0/3 unit 0 family bridge interface-mode access
set interfaces xe-1/0/3 unit 0 family bridge vlan-id 124

Now add it to the uplink (that in this case has a layer3 instance on it as well):

set interfaces et-5/0/0 description "100G North"
set interfaces et-5/0/0 vlan-tagging
set interfaces et-5/0/0 mtu 9192
set interfaces et-5/0/0 unit 123 description "L3 testing vlan 123"
set interfaces et-5/0/0 unit 123 family bridge interface-mode trunk
set interfaces et-5/0/0 unit 123 family bridge vlan-id-list 123
set interfaces et-5/0/0 unit 124 description "L3 testing vlan 124"
set interfaces et-5/0/0 unit 124 family bridge interface-mode trunk
set interfaces et-5/0/0 unit 124 family bridge vlan-id-list 124
set interfaces et-5/0/0 unit 100 description "VLAN100 Layer3 Peering"
set interfaces et-5/0/0 unit 100 vlan-id 100
set interfaces et-5/0/0 unit 100 family inet mtu 9000
set interfaces et-5/0/0 unit 100 family inet address 10.100.100.1/30
set interfaces et-5/0/0 unit 100 family inet6 mtu 9000
set interfaces et-5/0/0 unit 10 family inet6 address 2001:fd8:a100:100::1/64

Now check your bridge table:

buraglio@mx480> show bridge domain
Routing instance        Bridge domain            VLAN ID     Interfaces
default-switch          vlan-123                 123
                                                             et-5/0/0.123
                                                             xe-1/0/0.0
                                                             xe-1/0/1.0
default-switch          vlan-124                 124
                                                             et-5/0/0.124
                                                             xe-1/0/2.0
                                                             xe-1/0/3.0

That’s it. Pretty straightforward.