After more searching, it was looking like a problem with ffmpeg defaulting to yuv444p and firefox not supporting it, but when I encoded it with yuv420p, it sometimes works when loaded directly, and still says it's corrupt when viewed in a web page:

The file: http://www.eternalmachinery.com/ecar/tbucket18crf55yuv420p.webm

The web page it's embedded in: http://www.eternalmachinery.com/ecar/

When I first tried to view it directly, it didn't load. After rebooting my phone, and loading it directly, it played fine. Then, moments later, loading directly, it wouldn't load again.

This file was created with: ffmpeg -framerate 60 -i tbucket%04d.png -c:v libvpx-vp9 -crf 55 -b:v 0 -pix_fmt yuv420p tbucket18crf55yuv420p.webm

And that yuv420p version still works fine with chrome on android, and firefox on my desktop.

Summary: firefox on android does not work with vp9 *or* yuv444p chroma subsampling.

Okay, the reason the webmproject.org file worked and mine didn't is theirs was encoded with vp8, and I used vp9. I then checked the combinations, and verified that firefox on android does not work with vp9 *or* yuv444p. So I guess this is two bugs in one.

The ffmpeg command line that worked was:

ffmpeg -framerate 60 -i tbucket%04d.png -c:v libvpx -crf 55 -b:v 0 -pix_fmt yuv420p tbucket18crf55yuv420pvp8.webm

The important parts being:

"-c:v libvpx" to specify the vp8 codec (the older codec for webm), and

"-pix_fmt yuv420p" to specify 4:2:0 chroma subsampling instead of full chroma 4:4:4: https://en.wikipedia.org/wiki/Chroma_subsampling#4:2:0

ffmpeg defaults to 4:4:4 because the input (png images) is 4:4:4.

I determined the difference between the two files with: ffmpeg -hide_banner -stats -i filename

The results included:

Input #0, matroska,webm, from 'big_buck_bunny_trailer_480p_logo.webm':

   Stream #0:0: Video: vp8, yuv420p, 852x480, SAR 1:1 DAR 71:40, 50 fps, 50 tbr, 1k tbn, 1k tbc (default)

Input #0, matroska,webm, from 'tbucket18crf55yuv420p.webm':

   Stream #0:0: Video: vp9 (Profile 0), yuv420p(tv), 800x450, SAR 1:1 DAR 16:9, 60 fps, 60 tbr, 1k tbn, 1k tbc (default)

All these combinations (vp9 + yuv444p) work on chrome on android and firefox on my desktop.

The phone I'm testing this on is an LG G6.