www/firefox52 and h.264/mp4

[Cág <ca6c%bitmessage.ch@localhost>]

Hi everybody,

I'm still using www/firefox52 because it doesn't require
Rust and GTK3.

There is sound, WebM and Ogg videos are playable, but
mp4's, for some reason, are not.  Installing gstreamer1
and the plugins (base, good, ugly, libav) doesn't change
it.  I think Firefox should be able to play them because
multimedia/ffmpeg3 is installed, since it is a dependency.
I also think that it used to work in the past.

The following options are used:
alsa gtk2 mozilla-jemalloc webrtc

Does someone know how to solve this problem?

Thank you!

--
caóc