While thinking about posting an bug to mozilla for the “xmlrpc” URL scheme, I found that someone beat me to it.
Poking around a bit more, I came across Necko documentation that showed Mozilla’s library has a generic url scheme that looks like this:
scheme://username:password@host:port/directory/filebasename.fileextension;param?query#ref
So I’m changing the BNF for the protocol again so that it looks for “?” before the parameter list instead of “,”.