www/firefox: update to 56.0 (rust required)

This is slightly early but due to recent PORTREVISION bump there's no
point doing QA for 55.0.* anymore.

Changes: https://www.mozilla.org/firefox/56.0/releasenotes/
PR: 221335
Security: 1098a15b-b0f6-42b7-b5c7-8a8646e8be07
MFH: 2017Q3 not possible: requires r447450 and r450556