Inactive tabs can launch dialog boxes so they appear to be displayed by a web site in another tab. This can be exploited by a malicious web site to show a dialog box, which seems to originate from a trusted web site. Successful exploitation would normally require that a user is tricked into opening a link from a malicious web site to a trusted web site in a new tab.
Darn summary got screwed up. I'm not sure if this was posted yet, but it didn't show up in Bugzilla
bug 124750 is the second part of it, but I'll dupe this to the meta bug. *** This bug has been marked as a duplicate of 262887 ***