I uninstalled Winamp from [C:\Program Files\Winamp]. There remained over 600 registry key values containing data with the now obsolete path to winamp.exe. Of nine Windows file associations I checked, the file extension.apl was no longer associated with any application. The file extension .m2z no longer appeared at all. File extensions .amf, .far, .m3u, .mat, .rf64, .stm, and .xi continued to be associated with Winamp at its obsolete path.
I then installed Winamp in [C:\Winamp]. Many registry key values still contained data with the now obsolete path to winamp.exe. File extensions .amf, .far, .m3u, .mat, .rf64, .stm, and .xi still continued to be associated with Winamp at its obsolete path.
After manually updating my Windows registry, changing the path to Winamp from [C:\Program Files\Winamp] to [C:\Winamp], I still had the originally-reported problem. I resolved that problem (per the suggestion of someone in the mozilla.support.seamonkey newsgroup, not here). As noted in my original message here, the problem was with the URI [http://kuscstream.org/mp3/kusc128.m3u]
. I captured the incoming header when selecting a link to that URI and noted the MIME type was audio/x-mpegurl. In the SeaMonkey preferences, I marked the handling of that MIME type as "Ask Me"; it had been Winamp. I then marked the handling of that same MIME type as "Winamp", which resolved the problem.
NOTE WELL: The handling of MIME types is inherent in Mozilla's Gecko core, which is common to all Mozilla-based browsers, including Firefox and SeaMonkey. The fact that I use SeaMonkey thus does not remove this problem from afflicting Firefox.