Firefox has a refresh feature, which resets some non-essential data to help fix and prevent issues you have using Firefox. Your Firefox Sync data does not get reset, but there may be cases when resetting that data will help fix issues. It will erase data on the server, but don’t worry; your local data will stay intact.

1. Open the Firefox Settings page (about:preferences), and select the Firefox Account panel.
2. Click Disconnect, then click Just Disconnect to make sure you’re not signed in.
   
3. Click Sign in.
4. Enter your Firefox Account email address, then click Continue.
5. Click the Forgot password link.
   
6. Enter your Firefox Account address, and click Begin Reset to receive a password reset email.
7. Follow the instructions in your reset email to change your password.

After you’ve reset your password and reconnected Firefox to your account, your local data will be uploaded to the server.

http://ilias.ca/blog/2018/11/how-to-fix-firefox-sync-issues-by-resetting-your-data/