www/firefox: update to 51.0 (rc1)

More candidates are likely to come before the official announcement (aka
the rush hour) scheduled on 2017-01-24. This one intended to pick up upstream
security fixes earlier while looking for downstream regressions.

Changes: https://www.mozilla.org/firefox/51.0/releasenotes/
Security: e60169c4-aa86-46b0-8ae2-0d81f683df09
MFH: 2017Q1 (piling up)