Both devices need to be online at the same time if you want to pair a device with the (3x4) 12-character code.
Sync needs to be set up and working on at least one (desktop) computer, so you can enter the 12-character code, provided by the device (computer) that you want to pair, in three fields to pair a device on the computer that has a working Sync setup.
Enter all details like the account name (email address) and password and the 26-character Sync key manually to connect to an existing sync account instead of using the 12-character code to pair a device if pairing isn't possible.
open "Tools > Options > Sync" or "Firefox > Set Up Sync"
click "Set Up Firefox Sync" to go to "Firefox Sync Setup"
click "I Have an Account" : Connect
click "I don't have the device with me" at the bottom to enter the account settings (email, password, sync key)
click the "Sync Options" button at the bottom to select what to sync