Hi
One of the improvements with Firefox 4 is the ability to handle the new
WebM format.
It's true.
Firefox 4.0 Beta 2 will play these files and Firefox 3.6.8 won't.
With Firefox use File > Open File...
There's a sample file attached to try.
These files are also playable with the current version of VLC media player.
I think that these files are also supported with Opera and Chrome browsers too.
This is Firefox 4
And this is VLC

---------- Post added at 17:07 ---------- Previous post was at 16:59 ----------
I can't upload the sample file.
They are available to download from here:-
http://www.webmfiles.org/demo-files/