If you can stream it, you can download it!
When I watch a rather long videos, usually the file in the cache would only store 25,584 KB of the file. This numbers is the same no matter what long video I cache. And what rather interest me is, when i see the video in my browser. It still replay able to its full length.
First you can type about:cache in your firefox address bar. There you can see the Disk Cache Device information. It can show you how many entries in your cache, Maximum Storage Size, Storage in use, and a link to the cache directory.
Well one thing that i realize is, the size of 25,548. Is the size of half of the default Maximum Storage Size in firefox, which is 50 MB. So i tried to increase the Maximum Storage Size to 200 MB and refresh the video I'm watching, and finally able to get the full size video, which in my experiment is 29,418 KB.
I think firefox is limiting the size of a cache file into half of the maximum Disk Space. This because its almost imposible for a cache directory to only contains a single file. Almost all the time the directory will contains more than one file.
But that doesn't answer why i could re-play the full file in my browser ?
My guess is, it is stored in the memory. To prove it i try watch the firefox memory usage in windows task manager. And close the tab and reopen it again. The memory usage drop significantly, but not quite the same as the full size of the file. But i'm guessing that my hunch is correct.
The Temp will not allow copying of contents, however. I don't intend to do more than view the youtube video on a machine without internet connection, so I don't think it's infringement on copyright. I would gladly take the lower quality video instead, but it seems to be disabled on the youtube site. They are switching more and more to the high bit videos, which will make piracy a tempting prospect for some. That might be reason enough to limit the Cache size in the browser -- it uses the data from the Temp file for playback anyway. But they should retain the low-resolution copies for folks to use remotely for their own use. Or Seamonkey could simply expand the capacity of the Cache to accommodate the new realities -- and trust users not to use it for piracy...