Yes, we should keep the CSS sprites here. Here is some agenda for v1.1 hi-dpi branch : https://docs.google.com/a/mozilla.com/document/d/1ekF4TP9t0uIhUbqGHg6ZPNMPBAEbjqXoiC29sBwidao/edit. I will submit a patch for solving hi-dpi issue in message app this week (https://bugzilla.mozilla.org/show_bug.cgi?id=881155), but we still has more hi-dpi issue here...(https://bugzilla.mozilla.org/show_bug.cgi?id=881126)

Anyway, I think this patch is fine for both v1.1 & v1.1 hd. Using the rem here should have no relationship with device dpi because they are CSS pixel. The major changes for hi-dpi branch is forcing the image size with CSS pixel unit.