Another way of downloading web videos(mostly flv formats) or images(jpeg,bmp,gif etc) or any other web related data that you see on a website using Firefox,is to download the Cache Viewer add-on.After watching a video on youtube,for instance,immediately open the Cache Viewer which is under the Tools menu and click on the MIME Type.Keep clicking until you get some extension names such as

text/xml
video/flv
image/jpeg

Look for the rows with MIME Type video/…
Check the Last fetched date.The one closest to your current time is the Video you have just watched.
Right-Click on it and Save as Filename.flv
Filename can be any name you want(just in case someone is wondering :) )
(The original filename usually associated with online videos is “get_video”)
But do remember to put in the extension as .flv

Check the file by playing the saved file with a player that supports flash videos(or flv files) such as VLC Player or K-Lite Codec Media Player.
Just in case this is not the video you are looking for,then go back to the Cache Viewer and save the next closest file on the list.You will eventually get your video.

The reason why I have posted this method is that all these video grabbing softwares usually re-download the video from the site.On the other hand, you are only copying the file which is already there in your hard disk,stored in the temporary files folder of your web browser.
This trick is especially helpful for those who have low bandwidth internet connection.
For those who have high-speed connections,
PLZ DON’T FLAME ME!!!
:)

P.S. The above method DOES NOT work for STREAMED AUDIO.

You can also try Web Video Ripper and other products from the site: http://use4video.com/