Where can I find the downloaded videos on my hardisk after I've watched it in a web browser?
Yes the videos you watch on youtube will be placed in /tmp folder..The name of the video will be in the form of alphanumeric you have to rename it to .flv For example if the file name is fff2356xg you have to rename this into fff2356xg.flv
Note: Not all videos you watch will be stored in /tmp folder..As far as i know youtube will work
You can also save your youtube videos by adding save infront of the URL,For example if the youtube video is http://www.youtube.com/watch?v=0USn7eufXps you want to add save infront of the URL http://www.saveyoutube.com/watch?v=0USn7eufXps so that you dont want to watch the whole video,you can simply download it.
You can also install youtube-dl to download youtube videos,it is a simple command line utility to download youtube videos.
To install youtube-dl:
sudo apt-get install youtube-dl
Type youtube-dl http://www.youtube.com/watch?v=0USn7eufXps in terminal to download the video
Although the /temp folder is the most common place, keep in mind depending on the web site player and flash version, videos can be stored in the browser cache folder. If you are using Firefox, the cache folder is under ~/.mozilla/firefox/yourprofile/Cache. If you are using Opera, the cache folder is under ~/.opera/cache. If you are using Chrome, then the cache is under ~/.cache/google-chrome.
Sometimes locating these files in the cache is not so simple, because they might be stored under a folder inside the cache. What I do is filter the folders by date it has been modified.
Keep in mind some sites use rtmp protocol to prevent their video streams from being downloaded. In this case, you won't find any video files in the tmp or cache folder.