update www/firefox to 66.0.4 (rc3)

https://www.mozilla.org/en-US/firefox/66.0.4/releasenotes/

This includes the fix for "armagadd-on-2.0" (disabled add-ons due to an
expired intermediate code signing certificate):
https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/
https://support.mozilla.org/en-US/kb/add-ons-disabled-or-fail-to-install-firefox

MFH: 2019Q2 (blanket: web browser)