Sync isn't to be used as a backup. It only sync between devices. If another device doesn't have thise data there's nothing to sync to. And formatting usually wipes any local data gone. And I doubt you have a history to sync back too. So unless you used Firefox backup FAQ instructions there is no way to get your data back. When you did a new sync anything that was there was replaced by the new sync login.

Hello,

Seems I've misunderstood what it's for then, how come it has worked in the past then? The "old" device is still active on my account and should still have data associated with it surely? Thanks.

I should also mention that history and extensions have been synced, just not bookmarks. Thanks for your help so far.

FYI... SYNC IS NOT A DATA BACKUP SERVICE Sync info... https://support.mozilla.org/en-US/kb/how-do-i-set-sync-my-computer https://support.mozilla.org/en-US/kb/how-do-i-choose-what-information-sync-firefox https://www.mozilla.org/en-US/firefox/features/sync/ https://hacks.mozilla.org/2018/11/firefox-sync-privacy/ Sync is not a backup tool but it is a good sync/transfer/move tool https://support.mozilla.org/en-US/forums/contributors/707681