I converted some .mp4 files to .webm with Miro. They play in Chrome and Opera but they do not play in Firefox 16. Firefox has supported .webm since FF 4. I tested video.canPlayType() and it returns "probably". I know that the MIME types are correct in .htaccess. I downloaded the a test .webm file from html5rocks and uploaded it to the same server and it played fine in FF, which makes me think it must be the encoding. I tried converting the videos multiple times and still no dice. Can anyone think of what would cause the them to not work in FF? Is there a more reliable way to convert videos to .webm?
Chrome_ImF.webm <== works in FF
amber-miro.webm <== does not work in FF
costa-miro.webm <== does not work in FF
There seems to be no problem with the encoding, however there are some errors with the file format on the webm files you created with Miro. I suggest using ffmpeg.
I've tested amber.webm which you provided as a sample. I used ffmpeg to re-format it into webm, without encoding, using the commandline:
ffmpeg -i amber.webm -acodec copy -vcodec copy amber1.webm
And the resulting file seems to play well in Firefox.
To directly convert .mov to .webm via ffmpeg you can use:
ffmpeg -i amber.mov -qscale 0 amber.webm
-qscale 0 is meant to retain the highest quality. See the ffmpeg docs.