This add-on for Firefox is really useful to download the videos from any websites that you see. You don’t have to install any third party software. Go to Tools -> Add-ons and search for Embedded Objects and add this to Firefox. You many have to restart your browser to get this add-on activated. Go to YouTube and play a video, when the video starts to play you can see a icon in the bottom of your browser. Click that icon and you can see the file named something like ‘video playback’ select that file and click the download button to get the video to your computer.
See the video file that demonstrates how to use this add-on
Make sure the add on bar is selected. To do this go to View -> Toolbars -> select Addon Bar or ctrl + /