How to watch youtube videos offline without downloading them

Hi friends, today i got a new idea to watch you tube videos offline without downloading them. I got this idea when i was trying to download trailer of movie anjana anjani and my dial up network went down. Let's see how it works-
Open your firefox and clear your recent history and cache , Now open the webpage that contains video, watch the full video , Now close that webpage or not doesn't matter.
Open a new tab and write "about:cache" and copy the path of cache directory under the disk cache device. It will look like thisC:\Users\dell\AppData\Local\Mozilla\Firefox\Profiles\u2xv72lp.default\CacheNow find this cache file in your computer by typing it in my computer or as you find easy.
In cache folder arrange files by size , files will look like the picture below
Select the first and biggest file in folder because you have cleared your cache and the biggest file will be your video only and open it with VLC media player. No need to download, just watch and save your video.