HiDPI displays are becoming more and more common (not just Mac Retina, but on Windows too). Bug 1016548 and 1016543 tracking work in this area. Lightweight themes are an interesting case. We support and promote them out-of-the-box, and when you're using one they're _extremely_ visible as primary UI. But the actual themes are almost always from external community members, and so we can't directly make HiDPI themes available (other than creating a handful of our own, and encouraging others to do so). And most of the work will probably actually be to modify AMO to accept such themes. Given how visible and popular these can be, we should make it possible to create, upload, and us HiDPI themes. But I don't think this is the same priority as other HiDPI work, so I'm going to firefox-backlog- it initially.

Per policy at https://wiki.mozilla.org/Bug_Triage/Projects/Bug_Handling/Bug_Husbandry#Inactive_Bugs. If this bug is not an enhancement request or a bug not present in a supported release of Firefox, then it may be reopened.