I addition to the previous comments about Firefox add-ons, you can often find
the file in your tmp folder in your root file system. It will be a large file
with an alphanumeric name, often beginning with capital F, and it will grow as
the video is streamed, but it will disappear after you close the viewing window.
This does not always work but most often it does. I just drag the completed file
to my desktop and re-name it.