Here is what you can try: 1)Check your Firefox Sync settings:

  *Make sure that you are logged in to your Firefox account and that Sync is enabled.
  *Check that the correct account is selected under "Connected Devices" in the Sync settings.
  *Try resetting your Sync data by clicking the "Reset Sync" button in the Sync settings.

2)Check your internet connection:

  *Make sure that you have a stable internet connection. Sync may not work properly if your connection is slow or unstable.

3)Check for any issues with your Firefox profile:

  *Try creating a new Firefox profile to see if the issue persists. If the problem does not occur with the new profile, the issue may be related to your current Firefox profile.
  *You can try renaming or deleting the "profiles.ini" file in the Firefox profile folder to reset your profile settings.

4)Check for any issues with your device:

   *Make sure that your device has enough storage space to store your Firefox data.
   *Try clearing the cache and cookies in Firefox to see if that resolves the issue.

If none of the above does not work, you may need to restore your bookmarks from a backup. If you have a backup of your bookmarks, you can restore them by following these steps:

1)Open the Bookmarks manager by clicking the Library button (the book icon) in the toolbar, then selecting "Bookmarks" and then "Show All Bookmarks" from the menu. 2)Click the "Import and Backup" button in the toolbar and select "Restore" from the drop-down menu. 3)Select the file containing your backup bookmarks and click "Open". 4)Click "Restore" to restore your bookmarks from the backup file.

I hope this helps! Let me know if you have any other questions.