> but it was a bit long so I thought I'd watch it later by taking the file from my Firefox cache. > I watched some of this video on this website Was it raw format when it says undf format? I wonder if its the saving to webm from the webpage that makes it playable from the cache too? VLC player does indeed play it from the cache now! Having gone to the webpage again and watched some of the video again but not letting it finish downloading then saving it as a webm file elsewhere then Using VideoCacheView again to look in the cache again its appeared as a 20MB webm file. It was a 70MB file in my cache yesterday but in unplayable undf format. Other videos I fail to play with VLC from the cache say nothing with no time length.

I do come across undf format every now and then, so it would have been good to play undf videos at times. Said 'try media player classic, better than vlc imo anyway plus it can use all sorts of codecs' I had Googled it earlier and this webpage

You should have just googled for 'undf'.

Vlc media player undf video