I’ve done it manually like described here: How-to: h264 etc. Support for Firefox (including ffmpeg install) and it is working for me.
What I’ve noticed are different directories to your script. Maybe that the point?
§ cat /etc/ld.so.conf
include /etc/ld.so.conf.d/*.conf
/usr/local/lib
$ cat firefox.conf
export LD_LIBRARY_PATH=/usr/local/lib
FFMpeg Conf is here:
/etc/ld.so.conf.d/ffmpeg.conf