At the moment, when people try to use Launchpad to report a bug on a project that doesn't use Launchpad for bug tracking, they will be shown a page directing them to file the bug against a distribution source package of that project or on the upstream bug tracker (with a link thereto). See https://bugs.launchpad.net/thunderbird/+filebug for an example.
Along with (or maybe instead of the link) to the upstream tracker, Launchpad should provide a link to the bug filing form for the project on that tracker (e.g. https://bugzilla.mozilla.org/enter_bug.cgi?product=thunderbird) so that people can click through directly to that form. This is particularly useful to triagers and the Ubuntu bug squad who frequently file bugs upstream.
This bug requires bug 300641 and bug 300686 to be fixed first.
Infrastructure for this landed in r7655.