This occurs with Seamonkey 2.13.2. When pressing the fullscreen option, the player disappears and the console shows:
"Timestamp: 09-11-12 11:17:13
Warning: Request for full-screen was denied because full-screen API is disabled by user preference.
Source File: https://commons.wikimedia.org/wiki/File:Maschine-stadt-luzern.ogv
Line: 0"
Version: master
Severity: normal