www/firefox: update to 52.0 (rc2)

Changes: https://www.mozilla.org/firefox/52.0/releasenotes/
Changes: https://hg.mozilla.org/releases/mozilla-release/
Security: 96eca031-1313-4daf-9be2-9d6e1c4f1eb5
MFH: 2017Q1 (piling up)