Pour utiliser les extensions Android, vous aurez besoin de Firefox pour Android. Pour découvrir les modules complémentaires pour Firefox, veuillez visiter cette page.
The extension does what you are asking for. However, your history of watched videos cannot come out of nowhere. Thus, if you did not synchronize the extension with your browser history (your browser history could be incomplete) or YouTube (you may not be logged in) then you will not see old videos marked as watched.
