When CSS `height` is defined with the `calc` function, the value of `padding-bottom` is ignored, i.e.:
Platform: Firefox Nightly 25.0a1 (2013-06-25)
Upon further investigation, I've found that the use of `calc` is irrelevant. Specifying "overflow" triggers this behavior, which as bug 748518 documents, is expected.
*** This bug has been marked as a duplicate of bug 748518 ***