www/firefox-esr: update to 38.3.0

build2 is currently tagged as release in mozilla-esr38
https://wiki.mozilla.org/Releases/Firefox_38.3.0esr/BuildNotes

Changes: https://www.mozilla.org/en-US/firefox/38.3.0/releasenotes/
MFH: 2015Q3 (after release announcement)