hi remonator, thanks for providing the log file - it shows the error code "invalid-client-state". according to this comment by a sync developer you'll probably have to reset the password of the account in order to resolve this error condition:

"The invalid-client-state is supposed to signal that you're logging in with an old password, and should re-login with your updated password. If you're in this state but are using the currently-valid FxA password, the only way out of it is to do a password reset. "

^{https://bugzilla.mozilla.org/show_bug.cgi?id=979009#c6}

Hi philipp, I changed my password and restarted Firefox, but unfortunately the error did appear again...

You can also try to disconnect from Sync and reconnect and possibly delete the signedInUser.json file in the profile folder with Firefox closed.

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

I deletet my account and registered with a new one. I also deleted the signedInUser.json file. Now I'm signed in with all my devices again and it seems to work! Thank you all!

FIXED: I had the same problem. Every time I close Firefox it would clear the saved Sync password. I unchecked that function here:

FireFox Tools > Options > Privacy > Clear history when Firefox closes > Settings > Saved Passwords.

However, when I open Firefox up again the preference change I just made would reset back to clear saved passwords.

To fix that, I had to clear all the content from the User.js file in my Firefox profile folder. Details are in this article.

https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save#w_specific-preferences-are-not-saving

What a pain it was to find/fix this. Firefox should make managing User profiles much simpler for the average user. They shouldn't have to perform surgery in any files like this.

A user.js file is not present by default in the profile folder, so if this file is there then you usually have created this file yourself (it is possible that other software creates this file).

The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

I did not create the user.js file. I didn't know it existed until I had to troubleshoot why I kept having to re-enter my Firefox Sync password ever time I opened Firefox. I only learned about it on the Firefox support site as a possible fix. Apparently some other (unknown) software created it w/o my knowledge creating havoc with Firefox Sync authentication. So in my case, clearing the user.js file was the FIX. Good to know the user.js file is not there by default.