Did you connected with your Firefox SYNC Account?

When you click "SYNC Now" what happens?

First of all, the Firefox Sync service isn't intended to be a "backup medium" or a "Cloud". It is made to be used to Sync Firefox data and preferences among multiple devices.

That said, when you initially set up Sync did you save your Sync Key as the setup screen told you to do?
(see the 1st screenshot)
There are two buttons, one to Print and the other to Save. Without your Sync Key your data is useless and can't be retrieved from the Sync servers.

If you do have that "key", did you have any problems with the Add a Device screen when you setup Sync on the new OS installation?
(see the 2nd screenshot)
If you aren't setting up an additional device, the hyperlink at the lower-left = I don't have the device with me needs to be selected to get to where you can enter the Sync Key - the Sign In dialog (3rd screenshot).

You also need to check the Sync Options and make sure to select "Replace all data on this device with my Sync data".

If you have regenerated a new sync key the all data on the sync server has been removed.

• https://support.mozilla.org/kb/ive-lost-my-firefox-sync-account-information