If I turn on the HEVC transcoding option on in the Plex settings, and watch a video that uses the Transcoding, Firefox will have a very small black bar at the bottom (might have also impossible to see black bar on the right to keep the aspect ratio). No issues when using x264. I don't know if this is a Plex or Firefox issue, but it doesn't show up with other players/browsers that I've tried. The attached screenshot is taken from the bottom of the video by using PiP against white blackground.