Some previous version had this thing where a buffered video was saved in some folder named cache or something..i did it once in the older versions but i can't find it in the new versions.
See the about:cache page.
See also this extension:
Try this way (for youtube) :
1. as the video run from youtube click the share button and copy (right click and copy on mouse) the link
2. in http://keepvid.com/ paste the url /link that you copy above and click download
3. as loading java applet, probably you can see warning window(the application's digital singature...........application?) click run
4.select 1 of the links/quality)
(like ›› Download FLV ‹‹ - 240p or ›› Download FLV ‹‹ - 360p etc)
5. and in the opening window select save file
6.the download begins to your selected folder.
to do the above you do not have to wait all video to fully buffered because you have the link.
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