Installing the Video Download add-on for Firefox  
  1. After the installation of Mozilla, right click on the gray area of the Firefox Window.  A menu will open like the one below.  Check the "Menu Bar" as pictured below.  
  2.  From the "Tools" menu, select "Add-ons"  
  3. In the search window (below), type "video download"  
  4. Using the "Install" button, choose the appropriate video downloading add-on.  It is usually the first on the list.  Choose the "YouTube" add-on with the red arrow.  
  5. After closing Firefox and reopening the program, open YouTube and find a video to download.