As you might know thumbnails in Thunar are generated with tumbler. The
tumbler package in Fedora however cannot include the tumbler-ffmpeg
plugin. By popular demand I now rolled an add-on package called
tumbler-freeworld.
If you are a packager in RPMFusion, please review the package at
https://bugzilla.rpmfusion.org/show_bug.cgi?id=1823
Fedora's Xfce users will appreciate your help.
Regards,
Christoph