To manage your sync settings in Firefox, click your profile picture icon in the top-right corner and select "Sync Settings" ...

If any of these options are successful, you can access OneDrive and view the Sync and backup status of your data in Settings.

Make sure all folders you wish to sync are checked. To do this, right-click on the white OneDrive for cloud icon and click Settings. Select the accounts tab and click ‘Chose Folders’.