This add-on is the one of my favorites.
I've used it for almost 10 years and for the most part, it does a good job of downloading videos via the Firefox browser, mainly flv and mp4 video, but it works for mpeg, wmv and jpg image files as well.
It's free, never had any issues with malware or adware.
It is almost all that I wish from down-loader, it is very useful and easy to work whit it.
I truly recommended it to everybody.