In one of our earlier article we have introduced a Bookmarklet, PwnYouTube, intended for downloading videos from YouTube, Vimeo, DailyMotion etc. It is easy to download YouTube videos using this Bookmarklet even though the download options are limited.
On the contrary DownloadHelper is an add-on for Firefox and have number of options. Install this add-on from here and restart Firefox. When you are landing on a page containing links to images or movies, DownloadHelper icon gets animated and you can download some or all of the files at once. It also allows you to download files one by one.
To get more options like supported sites, modify your preferences etc. right-click on the icon.