I think you have to specify that it is a video file in Firefly.
stolen from turbo’s post:
celia:~# sqlite3 -column -header /var/cache/mt-daapd-test/songs3.db
SQLite version 3.3.8
Enter ".help" for instructions
sqlite> update songs set has_video=1,type='m4v',codectype='mp4v' where path like '%.mp4';
sqlite> select id,title,type,codectype,has_video from songs where path like '%.mp4';
id title type codectype has_video
4 Filename1 (mpg-mp4).mp4 m4v mp4v 1
9 Filename2 (avi-mp4).mp4 m4v mp4v 1
10 Filename3 (mp4-mp4).mp4 m4v mp4v 1