Interesting that I just had the same experience with webm files, but not with ogg, a year later on FF v.33. I posted about it here...
Why is Firefox HTML5 video jumping to the end?
Some idiot with nothin better to do voted me down, but who cares... the point is to share experiences and solutions. I'm sure this is a firefox/mozilla issue, because the same pages and files play fine in Chrome. For now, my solution is to pre-detect Firefox, and when detected block the webm from firefox, and force it to look at my ogv file instead. The way i do that is to force the webm source, which comes before the ogg source, to be commented out as the page is rendered, when ff is detected. I'd like to try adding correct codecs for my webm files, but in a pinch what I did is a good stop gap solution. I posted it here, in response to someone else's issue with webm files on FF.
not all webm files are playing in firefox