Shared components used by Firefox and other Mozilla software, including handling of Web content; Gecko, HTML, CSS, layout, DOM, scripts, images, networking, etc. Issues with web page layout probably go here, while Firefox user interface issues belong in the Firefox product. (More info)
For problems related to the HTML 5 media elements (<video> and <audio>) -- including WebM, MP4, MSE and EME issues. This would also typically include decoding problems in the codecs themselves (e.g. VP8, VP9, H.264, AAC) experienced during playback.