www/firefox: update to 50.0 (rc1)

More candidates are likely to come before the official announcement (aka
the rush hour) scheduled on 2016-11-15. This one intended to pick up upstream
security fixes earlier while looking for downstream regressions.

Changes: https://www.mozilla.org/firefox/50.0/releasenotes/
Security: d1853110-07f4-4645-895b-6fd462ad0589
MFH: 2016Q4 (piling up)