Sync is not meant as a backup. While it is possible, you risk losing the data on the servers. Restore Bookmark Backups Firefox automatically makes backups of the bookmarks. These are stored in the '''bookmarkbackups''' folder. Open the '''Bookmarks Manager; ''' Press the '''Alt''' or '''F10''' key to bring up the toolbar, and select '''Bookmarks. ''' Hot key is '''<Control> ''(Mac=<Command>)'' <Shift> B. ''' Firefox 84+ '''<Control> ''(Mac=<Command>)'' <Shift> O. ''' Once the window is open, at the top of the page, press the button labeled '''Import and Backup. ''' Select '''Restore. ''' From the list, pick the file you want and click it. The backups are stored in a '''jsonlz4 file''' named something like this; bookmarks-2016-06-29_1424_enD1GTH ya0JmicnYWg4V4g==.jsonlz4 In this example; 2016-06-29 = date created,:pasteb 1424 = number of entries. https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them https://support.mozilla.org/en-US/kb/recover-lost-or-missing-bookmarks https://support.mozilla.org/kb/forum-response-recover-bookmarks-after-update

Is that the only backup you have ? Sync only has the last used synced bookmarks, so if you were connected to Sync during or after restoring this backup then the bookmarks on the Sync server are likely already affected. Aren't there older backups available in the bookmarkbackups folder you can use ?