This occurs with software and VAAPI:

VAAPI:
MOZ_LOG="PlatformDecoderModule:5" LIBVA_DRIVER_NAME=i965 MOZ_ENABLE_WAYLAND=1 mozregression --launch 0c7df6f9b0c1 --pref gfx.webrender.all:true widget.wayland-dmabuf-vaapi.enabled:true widget.wayland-dmabuf-video-textures.enabled:true media.ffvpx.enabled:false -a https://bug1619882.bmoattachments.org/attachment.cgi?id=9149605 -P stdout

0:28.87 INFO: b'[Child 18955: MediaPDecoder #1]: D/PlatformDecoderModule AudioTrimmer[0x7f63af439ba0] ::HandleDecodedResult: sample[1021678,1044897] (decoded[1021678,1044897] no trimming needed'
0:28.87 INFO: b'[Child 18955: MediaPDecoder #4]: D/PlatformDecoderModule Created new WaylandDMABufSurface UID = 4'
0:28.87 INFO: b'[Child 18955: MediaPDecoder #4]: D/PlatformDecoderModule DMABufSurface: creating surface UID = 4'
0:28.87 INFO: b'[Child 18955: MediaPDecoder #4]: D/PlatformDecoderModule DMABufSurface: deleting dmabuf surface UID = 4'
0:28.87 INFO: b'[Child 18955: MediaPDecoder #4]: D/PlatformDecoderModule DMABufSurface: VAAPI releasing dmabuf surface UID = 4'
0:28.87 INFO: b'[Child 18955: MediaPDecoder #4]: D/PlatformDecoderModule DMABufSurface: VAAPI locking dmabuf surface UID = 4'
0:28.98 INFO: b'IPDL protocol Error: Received an invalid file descriptor'
0:28.98 INFO: b'IPDL protocol Error: Received an invalid file descriptor'
0:28.98 INFO: b'IPDL protocol Error: Received an invalid file descriptor'
0:28.98 INFO: b'IPDL protocol Error: Received an invalid file descriptor'
0:28.98 INFO: b'[Child 18955: MediaPDecoder #3]: D/PlatformDecoderModule AudioTrimmer[0x7f63af439ba0] ::operator(): sample[1044897,1068117] no trimming information'

Can you attach full log please?
Thanks.

Gnome Wayland, Debian Testing, Mesa 20.0.7, Intel Iris Graphics 6100 (BDW GT3)
Played the whole video and closed Firefox.

MOZ_LOG="PlatformDecoderModule:5" LIBVA_DRIVER_NAME=i965 MOZ_ENABLE_WAYLAND=1 mozregression --launch 0c7df6f9b0c1 --pref gfx.webrender.all:true widget.wayland-dmabuf-vaapi.enabled:true widget.wayland-dmabuf-video-textures.enabled:true media.ffvpx.enabled:false -a https://bug1619882.bmoattachments.org/attachment.cgi?id=9149605 -P stdout > vaapi.txt

MOZ_LOG="PlatformDecoderModule:5" MOZ_ENABLE_WAYLAND=1 mozregression --launch 0c7df6f9b0c1 --pref gfx.webrender.all:true widget.wayland-dmabuf-video-textures.enabled:true media.ffvpx.enabled:false -a https://bug1619882.bmoattachments.org/attachment.cgi?id=9149605 -P stdout > software.txt

While VAAPI seems to work until the end, software decoding seems to fall back:
From comment 4:

0:27.88 INFO: b'IPDL protocol Error: Received an invalid file descriptor'
0:27.88 INFO: b'[h264 @ 0x7fcb772c0800] nal_unit_type: 1(Coded slice of a non-IDR picture), nal_ref_idc: 0'
0:27.88 INFO: b'[Child 21210: MediaPDecoder #1]: D/PlatformDecoderModule DMABUF/VA-API Got one frame output with pts=520000dts=560000 duration=40000 opaque=-9223372036854775808'
0:27.88 INFO: b'[Child 21210: MediaPDecoder #1]: D/PlatformDecoderModule Got one frame output with pts=520000 dts=560000 duration=40000 opaque=-9223372036854775808'

My naive guess is that it could be bug 1634213: https://webglsamples.org/aquarium/aquarium.html is gray when widget.wayland-dmabuf-webgl.enabled is true. Debian Testing distributed the regression: I currently have Mesa 20.0.7. The fix was released with 20.0.8 last Thursday. (Ctrl+F: Firefox)

Likely unrelated to Mesa. Got the 20.1.1 release today which fixed WebGL.

Now with recent changes from bug 1619523 I can see these errors on X11/EGL as well. They appear either all at once, or during video playback. Then video freezes. Exact behavior seem to be quite random. It may freeze early into video, or it may happen later. Sometimes these errors appear in bursts, and at time I can see all-green frames.

Unfortunately, I can't capture a video with all-green frames that appear when "IPDL protocol Error: Received an invalid file descriptor" appear later. But there is a capture of a video freezing.

It's a dupe of Bug 1645671.