Sorry, your data isn't recoverable from the old Sync system when using the new version of Sync. The method of connecting is different than it was with the old system, which used a "key" in addition to the email address and the account password. When upgrading a Sync account the Firefox data needs to be on the device that was updated from Firefox 28 to a later version when connecting to the Sync server the first time. That is because the new version sends the data to a new Sync server using the same email address and password, but the new "key" isn't the same as with the older version - it is generated differently than before. Here are the instructions for updating to the new Sync: Sync your Firefox bookmarks, history, passwords and more

In conclusion, no version of Sync was intended to be used as a backup medium for one device, it was intended to synchronize Firefox data between multiple devices. Some users have made it work with one device, but not when the last Sync was made with the older version and then the user attempted to recover their data with the new version and a new operating system or a reinstalled operating system - that has no Firefox data.

u171098 said

I upgraded a linux netbook from Ubuntu 12.04 to Mint 17 and was unaware of the changes to sync that occured after FF28. I failed to unlink the old firefox and the clean install of Mint17 had the new sync version 29? I guess. How do I unlink the old account so I can sync the new install?? The account had only one machine syncing to it.

I found a procedure for downgrading to FF24ESR which I did. I used the sync key I had previously (6 months ago) printed out and saved (fortunately) from the old ff version and was able to recover the old synced data. Next, I unlinked the device from ff24esr, then upgraded to FF35, resynced with the account name and password and recovered all the old data to the new FF, whew... The wife doesn't have multiple platforms w/ FF, she didn't use it that way. This was a painful way to learn about the new FF sync after not being able to recover my wifes browser data during a change of OS.. she just expects it to always work, If only it were that simple.. I do appreciate all the work that gets done by mozilla but, agree with the ESR users and hope the update cycles are more limited..