Sync is intended to synchronize Firefox data across multiple devices. It should never "overwrite" any data, but always append or merge data from other devices. That said, "I used FF sync about 2 months ago" and the lack of mention of a 2nd device leads me to think that you were using Sync as a "backup medium"; not the intended purpose of the service. And, Sync should never pickup the same device and the exact same Firefox Profile as a "2nd occurrence" of the same device / Profile. If Sync was disconnected for that whole 2 months, that could have played a part in your issue. Sync is made to run unattended and to constantly "tweak" any differences in data each time Firefox is launched. One last thing, the Backup feature and Restore option shouldn't be mentioned "in the same breath" as Sync, the Backup JSON and JSONLZ4 files '''don't merge''' bookmarks - those files will overwrite any existing data. ''Export & Import in HTML format will append any existing bookmarks; but the Export needs to be done manually - the bookmarks.html file (if you have one in the Profile folder) isn't automatically updated daily as it was years ago.'' IOW, Sync is basically the opposite of Backup / Restore so far as appending vs. replacing in total. As far as "fixing" Sync for the issue you mention; sounds like user misunderstanding of how Sync works or user error by removing that device rather than editing the Device Name via Change Device Name ''(if that was a new Profile that was created or a Refreshed Profile since Sync was disconnected)''.