Daring Fireball: Creating Ogg Theora Files on Mac OS X With ffmpeg2theora – John Gruber points out that this step is necessary, since:
To use the HTML 5 <video> tag in Firefox 3.5, you need video files encoded in the Ogg Theora format. Apple doesn’t support this format at all, so you can’t just export Ogg files from QuickTime like you can with H.264/MPEG-4.
… and …
ffmpeg2theora is the one tool I found that simply just works for transcoding to Ogg Theora.