Try to delete the signedInUser.json file in the Firefox profile folder and delete the Weave folder (Sync was formerly named Weave). If that isn't enough then delete the prefs.js files to reset all prefs and maybe also delete the compatibility.ini file to reinitialize the profile.

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Atanasoff said

Hi - Sync keeps telling me it has been unable to sync for 14 days. I've tried a manual 'sync now' to no avail. I did try a 'Reset Firefox', but that seems to erase things I was working on (saved sessions), so restored using MzBackup. Here's the sync log, it looks the same as before. I've removed some disabled extensions, no change. What's causing this and how can I fix it? I'd prefer not to do another Reset, but I have the old profile and could probably (?) dig out the stuff I need if there's no other option. 1434904335309 Sync.ErrorHandler DEBUG Flushing file log. 1434904335311 Sync.BrowserIDManager ERROR Background fetch for key bundle failed: No keyFetchToken 1434904335311 Sync.BrowserIDManager ERROR Could not authenticate: No keyFetchToken 1434904335313 Sync.SyncScheduler DEBUG Clearing sync triggers and the global score. 1434904335313 Sync.SyncScheduler DEBUG Next sync in 3600000 ms.

It took several steps, but is now working. It was necessary to delete the prefs.js files, sign in to sync again (best not to forget what account you use for that), waitaminit, then check the log. Something or other corrupts some files is the profile: there's a series of files with names similar to content-prefs.sqlite-1.corrupt. In any case, it is syncing now. Tx for the tech tips!

You can remove the sqlite-#.corrupt files as they shouldn't be there and have no use as being flagged.

What security software (firewall, anti-virus) do you have?

A possible cause of such a file corruption is other software that inspects and tries to remove content from such a SQLite database file. Try to exclude the Firefox profile folder from your security software and other cleanup software if you have any.

I have Avast, paid version. I s'pose I can exclude the FF pref folder, but the system is rather vulnerable at it's running, uhm, windows xp. Yeah, I have another machine which only older than dirt rather than older than rocks, but this one is more comfortable to use... so it tends to have the last thing I was looking at open, so it gets used again. Hey, my $50 antique is still better guarded than the Federal personnel & security clearance system.

Well, sync is sinking again, though with a slight difference. I now have a banner at the bottom of the tab stating "Sync encountered an error while syncingn: Failed to connect to the server...." About:Sync-log shows a different entry than before: Before fix: a series of like error-nnnn.txt While fixing: error-sync-nnn.txt Day of fix: success-nn.txt Now: success-sync-nnn.txt, but with the above error message showing in each FF tab.

I did place a AV exclusion on the sqlite file.. and when I checked this just now it has disapeared from the AV exclude list. Hm. I don't see any sqlite.corrupt files tho. I'm re-entering the exclusions for content-prefs, but I don't think that'll fix whatever is now re-broken.