You first need to setup sync on the desktop computer. Then you can pair the mobile device via the 12-character code with the desktop computer. Did you ever create a sync account? *https://support.mozilla.org/kb/how-do-i-set-up-firefox-sync *https://support.mozilla.org/kb/add-a-device-to-firefox-sync 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 ---- *https://support.mozilla.org/kb/how-do-i-manage-my-firefox-sync-account *https://support.mozilla.org/kb/firefox-sync-troubleshooting-and-tips