your comment annoyed me enough to finally register here to be able to reply to you.
The OP wants to register the URL file type with firefox but configure the parameters passed to the program. This used to be possible in XP, but in Seven the file type association editor got revamped and is now missing this possibility. It is only possible to select the executable, not enter a full command line with additional parameters, e.g.
"C:\Program Files\Java\j2re1.6.0\bin\javaw.exe" -jar "%1" %*
I couldn't find anything helpful in the links you provided.