OK, the + symbol in the Firefox awesome bar is magic. It causes autocomplete to match only bookmarks with tags (which, since using XMarks and importing my delicious.com bookmarks, tends to be all of them).
Notice from the screenshot that in spite of the +, not everything that comes up is tagged abc: it finds abc in the title as well, hence the Nerd ABC's and ABC News links which are tagged something besides abc.
Does anyone know a way to make Firefox match tagged items ONLY by searching the tag, and not the title?
(btw, in preparing this screenshot, I noticed that abc matches "labcast" as well, but I am less concerned about that)