There are many extensions for in Firefox to download videos, but I was stuck so long in Chromuim Web Browser while I needed to download videos. So with my some internet surfing Flareget is my best alternative for video downloading. Let's say it is the alternative software of IDM in Ubuntu in the sense of speed. Flareget provides high speed for video downloading.
Follow the following steps:
1) Go to the link and select the download dile for debian package
In Terminal type sudo wget -c"www.flareget.com/files/flareget/debs/amd64/flareget_2.4-27_amd64(stable)_deb.tar.gz
This downloads .deb file for installing Flareget.
2) Double click the downloaded file and you will get forwarded to the Software centre and you will need to install from there. It won't take time to install because it is already downloaded .