I have a big problem with Firefox Sync on my Windows laptop. I constantly get the following error: "Sync encountered an error while synchronising: Unknown error. Sync will automatically retry this action." The error has always been there since I've set up Sync on Firefox on that computer. Looking at "about: sync-log" revealed that the issue is with the bookmarks syncing engine. Here are the relevant lines from the log: 1351062615899 Sync.Engine.Bookmarks DEBUG First sync, uploading all items 1351062616754 Sync.Store.Bookmarks DEBUG No node for GUID mobile; returning no children. 1351062616755 Sync.Engine.Bookmarks INFO 1122 outgoing items pre-reconciliation 1351062616791 Sync.Status DEBUG Status for engine bookmarks: error.engine.reason.unknown_fail 1351062616791 Sync.Status DEBUG Status.service: success.status_ok => error.sync.failed_partial 1351062616792 Sync.ErrorHandler DEBUG bookmarks failed: Component returned failure code: 0x80520012 (NS_ERROR_FILE_NOT_FOUND) [nsIFile.create] Stack trace: PU_B_saveBookmarksToFile()@resource://gre/modules/PlacesUtils.jsm:1882 < PU_B_create()@resource://gre/modules/PlacesUtils.jsm:2013 < PU_archiveBookmarksFile()@resource://gre/modules/PlacesUtils.jsm:1754 < <file:unknown> < _syncStart()@resource:///modules/services-sync/engines/bookmarks.js:350 < <file:unknown> < SyncEngine__sync()@resource://services-sync/engines.js:1282 < <file:unknown> < wrappedSync()@resource:///modules/services-sync/engines/bookmarks.js:214 < _sync()@resource:///modules/services-sync/engines/bookmarks.js:220 < <file:unknown> < WrappedNotify()@resource://services-sync/util.js:142 < <file:unknown> < Engine_sync()@resource://services-sync/engines.js:481 < <file:unknown> < WeaveSvc__syncEngine()@resource:///modules/services-sync/service.js:1393 < resource:///modules/services-sync/service.js:1286 < <file:unknown> < WrappedNotify()@resource://services-sync/util.js:142 < WrappedLock()@resource://services-sync/util.js:97 < <file:unknown> < _lockedSync()@resource:///modules/services-sync/service.js:1186 < resource:///modules/services-sync/service.js:1177 < <file:unknown> < WrappedCatch()@resource://services-sync/util.js:71 < <file:unknown> < sync()@resource:///modules/services-sync/service.js:1165 < <file:unknown> < <file:unknown> I've tried cleaning the sync data on the server, and "Reset Sync" various times, to no avail. Syncing add-ons and passwords works fine (I don't sync anything else). Please help! Syncing bookmarks is the main reason I'm using Sync at all!