Since, v 16.4 activation key feature is introduced and it will be the only way to use PRO features going forward, the old method is no longer supported due to cookie privacy concerns and misuse which results in degraded performance for others users.
1). Make sure you are using the latest addon version - https://www.yourvideofile.org/download.html
2). Open any Youtube URL and use "Settings" option from Download drop-down as shown below.
3). Use your payment email-id as your ACTIVATION KEY and save the settings. MUST NOTE : Make sure there is no mailto:, spaces or any extra characters attached to the payment email when pasting. Also, the email should be exactly as in your payment receipt, small and uppercase letters matters. Also, you need to REFRESH the video page and retry the download after entering the key. Windows XP and users having older Firefox versions need additional steps to make the activation stick in settings as described in steps at bottom of this article.
4). The PRO features should work now, if not do send-in an email to the original payment email with FULL payment confirmation receipt and issue/error details to get support. NOTE: The DONATE button will remain there but all features will get unlocked. Also, make sure you do not share the activation key with others as it will lead to auto-banning of shared key.
FOR WINDOWS XP USERS ONLY -->: Type about:config and then press Enter key in your url bar, search for - webextensions.storage.sync.enabled key, if not there or set to 'false' set to 'true as' shown below. After this resave the key, restart the browser and it will work.