FF15 gives a lot more details about memory consumption, because "compartments" (the way the JS heap is segregated) are a lot more fine-grained. In particular, compartments created by add-ons can now be identified. But note that this only covers some add-on memory consumption; the remainder is not reported separately, and doing so is difficult.

about:memory is quite intimidating, I'm hoping to implement soon a vastly simpler version that will give a simple per-tab breakdown and be understandable by normal users.