Try to delete the weave folder in the Firefox profile folder.

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

• Help > Troubleshooting Information > Profile Directory: Show Folder

You can also check for problems with preferences and try to rename or delete the prefs.js file and possible numbered prefs-##.js files and a possible user.js file to reset all prefs to the default values.

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences

Ok. I completely uninstalled Firefox, remove all .mozilla/firefox files and reinstall it. It works.

@cor-el Unfortunatelly your advice does not help, but thanks for reply :-)

I have never knowingly turned on Sync, on any machine, and only became aware of the feature when the error message appeared recently.

I've been "syncing" between two computers for several years by simply copying the profile from one to the other, both for Firefox and Thunderbird. Before copying, I make sure both machines are at the same update level, etc., and this has worked well for me.

Now I'm getting the "sync failed" message on the last target machine; I wonder if my profile-copying has screwed up Sync, and how I unscrew it. I'll start by reading up on Sync...