www/firefox: update to 55.0

This is the last release to support RUST=off. Later versions
remove non-Rust codepaths e.g., via encoding_rs.

Changes: https://www.mozilla.org/firefox/55.0/releasenotes/
PR: 216541 219963
Security: 555b244e-6b20-4546-851f-d8eb7d6c1ffa
MFH: 2017Q3