www/firefox: update to 51.0 (rc2)

Changes: https://www.mozilla.org/firefox/51.0/releasenotes/
Changes: https://hg.mozilla.org/releases/mozilla-release/pushloghtml?startdate=2017-01-17&enddate=2017-01-19
Security: e60169c4-aa86-46b0-8ae2-0d81f683df09
MFH: 2017Q1 (piling up)