sftp://, https://, wss:// are "secure" and ftp://, http://, and ws:// are "insecure". For some schemes, e.g. data:// and about://, there is no TLS-protected variant because URIs with those schemes do not indicate a network resource. The strict transport security service often rewrites URLs from the insecure variant to the corresponding secure variant. It needs to know, given the insecure scheme, what is the corresponding secure scheme. It seems likely that the implementation of bug 62178 will at least need to know whether or not a given nsIURI has a secure scheme. However, it might end up just asking the HSTS service to rewrite the URI to the secure variant.

It sounds nice in theory, but in practice every perceived use for this has been doable without it, and there are usually use-specific subtleties that have to be taken into account.