If media-plugins/gst-plugins-ffmpeg is not installed, HTML5 video doesn't work in www-client/firefox. The audio and controls work, but the video pane is blank. Please modify the Firefox ebuild to depend on media-plugins/gst-plugins-ffmpeg or media-plugins/gst-plugins-meta[ffmpeg]. Reproducible: Always
Maybe it ought to simply RDEPEND on media-plugins/gst-plugins-meta. This is what www-client/opera{,-next} do.
Is in overlay, will move to tree soon as we complete testing of other changes.
Dep has been adjusted in tree, thanks for reporting.