In case of issues with Sync you can try this:

Disconnect from Sync and Reconnect to Sync again. You can disconnect from Sync by clicking the e-mail address in the "3-bar" Firefox menu button drop-down list or go to "Options/Preferences > Sync".

Delete the signedInUser.json file in the Firefox profile folder and delete the Weave folder (Sync was formerly named Weave).

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

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

Remove an existing chrome://FirefoxAccounts entry in the Firefox Password Manager. 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.

Delete and re-create the Firefox account or change the Sync password.

Note that changing the Sync password will wipe all data stored on the Sync server.

Can you log in to the Sync account via this page?

• https://accounts.firefox.com/settings

Thank you.

As written above, I already tried do delete all my Firefox profile and that did not change anything.

I don't want to delete and re-create my Firefox account because I don't want to lose all my browsing history, cookies, sessions, password list etc.

I can't log in to https://accounts.firefox.com on Firefox. It says "Error 500". But I can on Chrome...

Maybe I should try to uninstall and reinstall Firefox, then.

I've just reinstalled Firefox.

Same bug. Error 500.

Did you do a clean reinstall and removed the Firefox program folder?

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen or hold down the Shift key) to see if that has effect.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

Do a malware check with several malware scanning programs on the Windows computer.

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

Please scan with all programs because each program detects different malware. All these programs have free versions.

Make sure you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  https://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender:
  https://windows.microsoft.com/en-us/windows/using-defender
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  https://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  https://support.kaspersky.com/viruses/disinfection/5350

https://support.kaspersky.com/viruses/utility

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked

Thank you. But I can't do the malware checks as this is my business computer and I don't have the admin rights.

The problem is solved, though. It seems that it was a professional network issue. I finally managed to connect to sync with a new Firefox profile stored locally in D: .

I had the same issue. The problem was "privacy.resistFingerprinting" set to true. It starts working after changing to false (default).