If you want to make principled decisions about codecs, then do them right.
This means that you either support H.264, or you don't.
If you do, you can do it half-assed – via a Flash plugin that's full of security holes and tends to crash the browser, or you can do it right – via OS support.
If you don't, then don't. Period. However, DownloadHelper and mplayer et al. are available for those who want to see the video, regardless of what Mozilla says. So is Chrome. Thus, the only outcome of adhering to The Principle is to further inconvenience and/or alienate users.
The current situation (pseudo-support via Flash) is an ad-hoc mess which follows no principle whatsoever.