On April 12, the ebuild with the alpha keyword for mplayerplug-in was removed. Please re-keyword 3.21 with alpha since this version of the plugin works fine for Mozilla 1.7, Firefox 1.5.0.4 and Firefox 1.0.8.
Both versions in the tree (3.21 and 3.25) have ~alpha. I'll look at marking 3.21 stable since has been ported to SeaMonkey and alpha has SeaMonkey stable.
Done. Thanks Mike!