Try again and download it from here https://addons.mozilla.org/en-US/firefox/addon/flash-video-downloader-youtube/

You should see a message about the file downloading and then a screen with a 3 second countdown should popup. When the 3 seconds are up you will get an "Install" button. Then Firefox will ask you to restart. Just click the restart button and when it's back up you should be all set.

An .XPI file is an installation file for Firefox etc extensions.

I would expect it to download and install directly from the add-on page *https://addons.mozilla.org/en-US/firefox/addon/flash-video-downloader-youtube/

If you downloaded the .xpi file instead of running it then open it in Firefox, e.g. from the File menu option of Firefox, and it should install.

You say Firefox is showing the extension as installed, so it presumably is installed already. Maybe you are trying to use it to download copyright items that it is not designed to work with.

Thank you for your help Verdi. I tried the download again. There is a window with Open and Save. I tried both buttons with the same result. XPI file downloads to Documents. No Install button. I click to restart Firefox and it restarts. Firefox shows extention as installed but it is not installed under Programs in my system, also no shortcuts anywhere. Thanks again.

Extensions are installed in Firefox only and should appear listed in Tools > Add-ons > Extensions

You should get a prompt to install the extension if you click the "Add to Firefox" button on the Add-ons page.

If you've saved the extension to a folder on the computer then either drag the extension in a Firefox window or use File > Open file (Firefox > New Tab > Open file) to install the extension.

If that doesn't work then see:

• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions

Thank you for your help John99. I did what you suggested. I had no luck opening this file. I have downloaded several add-ons from Mozilla add-on page. Usually there is a window with Run or Save options. Now there are Open or Save options. I tried both. I get no usual warning from Windows. FVD does not appear installed under programs. No shortcuts anywhere. Thanks again.

I did try the download myself, although not in Windows, and it installed without problems. It showed up as one of my extensions, and it added a button to my navigation menu bar.

Did you read the article cor-el linked to ?
Perhaps you should try some other arbitrary add-on and see if that will install. Is your problem with all installs of add-ons or only the FVD add-on ?