mozilla/pdf.js — pull request #5175 by Snuffleupagus (2014-08-12 10:59:51+00:00)
Title: Disable the "Go to First/Last Page" buttons when the first/last page is currently active

Since the "Previous/Next Page" buttons are disabled when the first/last page is active, I think that for the sake of consistency the "Go to First/Last Page" should behave the same way.