Firefox 20 added support for H.264 video on Windows, so I thought I'd try it out (toggle
media.windows-media-foundation.enabled in about:config). However, when using the HTML5 video player with Firefox, the letterboxing doesn't work right, as seen in the image to the right.
On a related note, clicking the big red play button didn't start the video either. It loaded the video, but then I had to click the little play button to actually play it.
Both of these things work in the Youtube HTML5 player, so I'm assuming this isn't a Firefox bug (or if it is, it can be worked around).
EDIT: I just noticed that the FAQ says this isn't supported for Firefox Beta. Even though this is no longer a beta feature, it is preffed-off, so maybe that still counts as "beta" in some way? If so, disregard this.