MFH: r452837

www/firefox: backport privacy.resistFingerprinting fix for User-Agent

If users are going to "pkg lock" due to addons better pretend to be an
ESR on the off chance JS malware is stupid enough to only check
User-Agent to filter out non-vulnerable Firefox versions.

Obtained from: upstream (Firefox 57)
Approved by: ports-secteam (swills)