Unfortunately this requires some design hints we don't have yet. I suspect we should add a "notitle" attribute to the row, and hide both the title and the separator boxes if the attribute is set (should be set if title and url are the same). The "bookmark without title but with tags" case must be considered appropriately to not look ugly...
Considered we are head down on the Quantum Bar rewrite, I don't think it's worth fixing this in the old implementation.
If you want to play with it, the code lives here
and it can be enabled in Nightly (or mozilla-central) setting browser.urlbar.quantumbar = true