It doesn't work in SeaMonkey 2.0, so I have sent an email to the author asking if it can be adapted to SM2.
Comments:Posted on 24 Nov 2009, 18:14 by BarryK
I compiled Firefox 3.5.5, just so that I can try out MediaPlayerConnectivity. But, I can't get it to work. I find the MPC configuration GUI confusing.
Posted on 25 Nov 2009, 6:51 by 466
I fixed an earlier version of betterprivacy to work
with seamonkey2 at one time. The current version of better privacy 1.45 is a good example of an extension that works in both firefox 3.5 and seamonkey2.
see also: http://netticat.ath.cx/forum/viewtopic.php?id=39