I submitted a bug before, which is still not done yet.
But one important thing that I didn't notice is this pkgbuild file needs further modifications.
IMHO, this it is out of date for unnecessary lines which make compiled pkg lack of locales and other features.

Here's simplified pkgbuild:

# $Id: PKGBUILD,v 1.39 2008/02/13 10:07:07 alexander Exp $
# Mantainer: Alexander Baldeck <alexander@archlinux.org>
pkgname=mplayer-plugin
pkgver=3.50
pkgrel=3
pkgdesc="mplayerplug-in is a browser plugin that uses mplayer to play videos from websites."
arch=(i686 x86_64)
license=('GPL')
url="http://mplayerplug-in.sourceforge.net/"
depends=('gcc' 'mozilla-common>=1.1-2' 'mplayer' 'libxi' 'libxt' 'libxpm')
makedepends=('xulrunner>=1.8.1.11-1' 'pkgconfig')
backup=(etc/mplayer-plugin.conf)
install=mplayer-plugin.install
source=(http://downloads.sourceforge.net/mplayerplug-in/mplayerplug-in-${pkgver}.tar.gz)
# mplayerplug-in.conf
md5sums=('7a9306c39f6550090c16b9c1d87e3168')
# 'a4b9d00153f3794feda4332662cc795e'

build() {
# temporarily use static paths during the transition to /usr
#export MOZ_PLUGIN_PATH=/usr/lib/mozilla/plugins

cd ${startdir}/src/mplayerplug-in

sed -e 's/mozilla-plugin/xulrunner-plugin/g' \
-i configure configure.in

./configure --prefix=/usr \
--with-faad --enable-gtk2 --enable-wmp \
--enable-qt --enable-rm --enable-gmp --enable-dvx
make DESTDIR=$startdir/pkg install || return 1

#make LDFLAGS="" || return 1
#mkdir -p $startdir/pkg/${MOZ_PLUGIN_PATH}

#install -m755 mplayerplug-in*so ${startdir}/pkg/${MOZ_PLUGIN_PATH}
#install -m755 mplayerplug-in*xpt ${startdir}/pkg/${MOZ_PLUGIN_PATH}

#mkdir -p ${startdir}/pkg/etc
#install -m644 ${startdir}/src/mplayerplug-in.conf ${startdir}/pkg/etc/mplayerplug-in.conf
}