Just we need 2 simple but yet powerful Mozilla firefox add-ons & they are
1. Downthemall add-on &
We need to install these add-ons in firefox, process is much simple and requires less then 5 mins to set tube downloder with resume capability. So below is the step-wise procedure.
Open Mozilla Firefox & then go to the page https://addons.mozilla.org/en-US/firefox/addon/downthemall/
[See below image, you will get this page]
Click on Download or Add to firefox button [Then small file in some kbs will begin to download, when it will complete a box will come with install button option]
Click on install [Then you will get option to restart firefox, click on it or you can manually close and open firefox] Now we have done part 1, we need to install one more add-on like this to get video quality options to download.
Go to the page
and repeat the steps 1, 2 & 3 to install this add-on [You must install this add-on also, to get download options]
Open youtube.com and start watching video which u want to download, If you have installed both the add-ons as mentioned as above then you can see three colored balloon icon in your firefox navigation toolbar. See small black arrow tip just in front of that colored balloon icon & click on that [You will get video quality options]
Move mouse to desired the quality option, then you will get some options at right side. Click on " Down them all!" You will get option box(To select save as, download location etc.) before download start.
Select folder where to save that video & then click on Start button, you will see you video is getting downloaded with good speed & resume capability.
Update: Now-days there are numerous Mozilla firefox addons to download tube videos you can grab anyone according to your choice,any addon installing procedure is same as shown in above]