There is a possibility the website might prevent you from using such plugins. If that is the case, it is recommended that you please go through the terms and conditions of the website. Also, it is recommended not to try to download any copyright content. These plugins are for you to download contents that are available for download form website for free.
If you use Mozilla Firefox there is an extension that you can use for downloading online videos. You can find this extension from the below mentioned link:
This extension is DownloadHelper. DownloadHelper does not break any real protection implemented on the sites. It gives easier access to available content that will be downloaded by your browser anyway. With some exceptions, downloaded videos must be kept on your disk for personal use and are not to be shown on other websites.
Please reply if you have any other questions.