There is definitely not shortage of video downloaders for the Firefox web browser. There are basically three types: Firefox add-ons that offer the video downloading functionality, Greasemonkey scripts that offer the functionality on specific video portals and programs that run on the operating system and integrate with the Internet browser.
NetVideoHunter (via Firefox Facts) is a Firefox add-on that works with multiple video portals including Youtube, Dailymotion and dozen others. The add-on is on the other hand not the typical video downloader that provides the download links either on the video portal itself or in one of the Firefox toolbars or status bar. It displays an icon in the status bar that displays a count of videos that have been found by the add-on.

This count stays even if the user leaves the website. A left-click on the number in the status bar opens a new window that displays all video names and urls plus two buttons to play or download the videos.
A click on Play will play the videos directly in the same window with the interesting option to change the size on the fly. There are also options to change the window to fullscreen mode and to smooth the video playback.
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- Download Avis from Youtube and other video sites
- Video Clip Download

I think this FF add-on worth a shot, thanks for the info.