Now I know everyone's first thought is that gecko-mediaplayer and gnome-mplayer have to many dependencies or that gnome-mplayer requires gnome. Well lets be frank. The only main requirement difference and dependency, if you are using Firefox, is that gecko-mediaplayer/gnome-mplayer require that dbus is present and running on the system. I've done some testing and dbus does not really add much RAM or CPU usage when it is not being used.
top - 08:10:30 up 7 days, 31 min, 8 users, load average: 1.08, 0.35, 0.16
Tasks: 3 total, 0 running, 3 sleeping, 0 stopped, 0 zombie
Cpu(s): 3.1%us, 2.8%sy, 0.0%ni, 93.7%id, 0.2%wa, 0.2%hi, 0.0%si, 0.0%st
Mem: 8179620k total, 6119660k used, 2059960k free, 608028k buffers
Swap: 2103292k total, 0k used, 2103292k free, 2992068k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1686 kdekorte 20 0 24992 3572 704 S 0.3 0.0 9:45.59 dbus-daemon
899 dbus 20 0 22996 2684 1236 S 0.0 0.0 0:09.42 dbus-daemon
1685 kdekorte 20 0 22220 508 300 S 0.0 0.0 0:00.00 dbus-launch
Tasks: 3 total, 0 running, 3 sleeping, 0 stopped, 0 zombie
Cpu(s): 3.1%us, 2.8%sy, 0.0%ni, 93.7%id, 0.2%wa, 0.2%hi, 0.0%si, 0.0%st
Mem: 8179620k total, 6119660k used, 2059960k free, 608028k buffers
Swap: 2103292k total, 0k used, 2103292k free, 2992068k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1686 kdekorte 20 0 24992 3572 704 S 0.3 0.0 9:45.59 dbus-daemon
899 dbus 20 0 22996 2684 1236 S 0.0 0.0 0:09.42 dbus-daemon
1685 kdekorte 20 0 22220 508 300 S 0.0 0.0 0:00.00 dbus-launch
As stated before gnome-mplayer does not require any gnome components, it originally did, but those components have been replaced with GTK/GLIB or other optional libraries. It can be configured to not use gconf or dconf. gnome-mplayer can be compiled to support GTK2 or GTK3 and since GTK is used by Firefox no additional library is added there.
mplayerplug-in does not support many of the features that gecko-mediaplayer does. The javascript is better, the support for Quicktime is better, and many of the problems that mplayerplug-in has are resolved in gecko-mediaplayer. So it is time for mplayerplug-in to finally go away.