Miro does not convert the entire file when converting to webm
I tried using Miro as well to transcode H.264 MP4 into VP-8 WebM so that Firefox would play using HTML5 rather than a Flash fallback.

But at least when the mp4 source was exported from Premiere then the Miro conversion did not work properly
it finished in my case, but the webm file would always start to play from the last second of the video for some reason (try it with VLC, for instance).

FireFogg Worked
My solution was to use FireFogg, which can encode into webm as well
http://firefogg.org/
Instructions here, just choose webmsd or webhd instead of ogg: Video - Dive Into HTML5