My understanding is that the 3_12_3_RC0 tag caused burning for some embedded builds (like mobile Firefox), so we need a tag with those fixes to update to on both trees.
windows CE is the platform that was burning, but obviously a tag update applies to all platforms.
Sorry I had missed this bug earlier. I've meanwhile filed 487712, landed a new NSS tag into mozilla-central, and we are waiting for tinderbox to cycle... Please feel free to move dependencies and mark as duplicate as needed.