debugging h264 support in ff57

[zinist]

Hello mp4 / h264 isn't working in my fresh ff57 build with a fresh firefox profile

On https://www.youtube.com/html5 "MSE & H.264" "H.264" are red, the rest is blue.

this system uses ffmpeg 3.4 that can play these formats just fine standalone with ffplay

how can I debug this issue.

In firefox, file -> open file:///home/user/big_buck_bunny.mp4 plays fine, yet when viewed on the page https://www.quirksmode.org/html5/tests/video.html it says "No video with supported format and mime type found" for the mp4 entry.

[therube]

Which Linux?
Is your FF from Mozilla or your distro?

[zinist]

handbuilt, mp4 works fine on a seamonkey build on this same system,
how can I debug what firefox 57 is doing, I need the failure output.

repeat note: the big bunny mp4 h264 file plays fine in ff57 when I use file -> open and point it to the local file on disk.

[therube]

It also isn't working in 54?

You've tested in a new, clean Profile? YES.

Wouldn't expect Safe Mode to make a difference, but try.

I don't know Linux.
Anything special required on that end?
Some symlink needed that you've set up for SeaMonkey, that isn't on the FF side?

[zinist]

I noticed a binary distribution of the 58 beta does work, but the binary of the 57 beta doesnt. So I'm building the 58 tree version now. Hope it'll work. Not sure what caused the issue about:buildconfig seems similar