I know I am replying to a thread that's months old, but I wanted to comment that the below helped me immensely. It was a pain to find out why videos weren't playing (FreeNAS11.2-STABLE) and eventually I found in the logs that ffprobe was failing, and it was related to "libx264.so.152 not found".
I didn't use the exact link below since there have been beta updates since then, but if anyone else stumbles across the problem with their FreeNas version of Emby not playing movies or video (music sometimes worked, and pictures worked), I recommend figuring out how to access the jails shell and updating to a beta release. I used https://github.com/M....0.83_amd64.txz
and while you are add it update to the newest beta it works flawelessly
service emby-server stop
pkg add -f https://github.com/MediaBrowser/Emby.Releases/releases/download/22.214.171.124/emby-server-freebsd_126.96.36.199_amd64.txz
service emby-server start