I had an old test box running XP and it was upgraded to Win7. In the process I lost my Firefox Sync with my main development box. Firefox Sync is wonderful, but it makes Byzantine bureaucracy look like a Manual for Dummies. It's full of blind alleys and circular mazes. Going to "about:home" and clicking on the "sync" icon at the bottom put me in an Alice In Wonderland world where I needed my Sync Recovery Key to log in, but couldn't get those magic numbers until I was logged in. Hmmmm. My solution:
1. Delete your account at https://account.services.mozilla.com
2. Recreate it with the same username and password, but be careful to get the "Sync Recovery Key" from the "Manage Account". The interface is a little odd, because it's assuming you want to change the key, not copy it. On the panel select "Cancel" after you have copied the key to a warm, dry place.
3. Went to my new device and started the process to pair the device.
Then it all worked. Good luck.