re: "How do I add a device to Firefox Sync?" There is no link for add new device.
The code for the new device is some 21 alpha numeric characters long. Only a few of the characters fit in the three boxes -- they do not all fit.
Alle svar (1)
The sync key is 26 characters and needs to be entered on the account page (I don't have the device), but if you have the code supplied by another device then you only have a code of 12 characters.