mplayerplug-in is a browser plugin that uses MPlayer to play
digital media from websites. This plug-in gives Mozilla the
ability to play media from a website the net without reading
the source html and getting the url manually. Media is played
embedded in the page or in a separate window depending on
how the author of the webpage intended the media to be seen.

You will need to have MPlayer installed (which is available at
SlackBuilds.org) in order to use this package.  Also, make sure
you have the SeaMonkey package (from stock Slackware) installed.