www/firefox: update to 53.0 (rc3)

More candidates are likely to come before the official announcement (aka
the rush hour) scheduled on 2017-04-19. This one intended to pick up upstream
security fixes earlier while looking for downstream regressions.

Changes: https://www.mozilla.org/firefox/53.0/releasenotes/
Security: 5e0a038a-ca30-416d-a2f5-38cbf5e7df33
MFH: 2017Q2 (piling up)