What sort of stuff is it better to file a report upstream about?

I was just wondering with some things if it is better to report them upstream (like a big new feature request or something) on the main Mozilla bugzilla site rather than here? I mean, if I file a report requesting a new feature, will the report be forwarded upstream, or how will the developers even get to know about it? Would it be that a patch is created here and then forwarded to them? It's just that I've made some pretty big feature requests here, and although I understand that you have a lot on your plate and that you can't look at every bug report, I was wondering if some things should be report upstream instead and if this was the right place for all types of report?