YouTube video you watch by entering its video URL. You can download these videos using IDM(internet download manager) but sometime we do watch that and after watching a half we think that we need to save it. If the video is fully loaded(light red seek line is full) .
You can click the IDM download icon to save it from cache (It will not download the file again just save it from cache in no time).
But what if you don’t have IDM in your computer or you haven’t loaded the full video and you want to save what ever loaded up to now. You can use Firefox’s local file cache to do it . Yes, it takes a little longer, but the plus is you don’t have to rely on another site just to download videos. Afterwards you can watch the video using the any player(must support flash video files, these days lot of players do support) of your choice.
Note before continuing: Yes, I understand there are applications like i mentioned above available that will do the same thing as what is instructed below, however it is better to do this manually rather than rely on an app which at some point will not work any longer.
It is little four step process.