Ah yes.That disreputable wizard whose name I've forgotten basically looted Sirius's mirror from his house along with some other stuff and Aberforth bought it.
IIRC there is something in the book when Harry is first given it - he gets the impression that it's something that will lead Sirius to come out of hiding if used, and he's determined not to risk that? - that tries to justify him putting it aside unopened, but it's very weak.
Not sure it's that weak though. They had contacted Sirius via the Floo network just before Christmas, which very nearly got him caught by Umbridge. Harry gets the mirror just after that, but he's determined not to risk Sirius' capture again. Probably justified in that Umbridge steps up her regime after christmas, including using veritaserum on students to get them to spill the beans.
Edit: For what it's worth, I found http://en.wikibooks.org/wiki/Muggles'_Guide_to_Harry_Potter/Books is a good place to look things up.