SOLVED: Movies dropping frames during direct playback

[TheDreadPirate]

Firefox does not play HEVC, period, AFAIK so it may have been transcoding. Your solution of remuxing tells me that the file was probably borked (not uncommon). Remuxing regenerates "packet time stamps" and various other non-video/audio components of the container that coordinate video/audio timing, which is often the cause of otherwise unexplained stuttering or dropped frames.