There are many plugins that are available to Firefox users that are capable of great many things. In some cases, as is the case of YouTube downloaders, there are many add-on to from which to choose. Not all add-ons are created equally, so to determine which one is best for your particular situation, you may find yourself downloading all the add-ons to test. Then things get confusing.
After downloading several different add-ons that enable downloading YouTube content, the Extensions among other installed add-ons will list what has been installed. I have highlighted several of these extensions.
Now, that the add-ons are installed, it may not be so obvoius has to which add-on is what control on the actual YouTube page. So, here I have highlighted and identified those controls on a YouTube page.
The reason this came up, a client had all of the add-ons installed and tested over time, however, forgot which one was the prefered add-on.
Of course, it could be easier to test one add-on at a time, too.