The minimum compatible version of Firefox has been lowered from `60.0`
to `56.2`. It was originally set to `60.0` because that is Firefox ESR,
which currently the minimum Firefox version with security updates.
However Waterfox is based upon Firefox `56.0`, and has backported
security fixes from the ESR release.

This change in minimum version requires no additional transpiling, and
doesn't affect any browser APIs we use.

It does introduce one additional warning in the `addon-linter` about
Firefox for Android `56` lacking support for browser action popups.
However there is no version `56.2` of Firefox for Android, so the
minimum version remains `57` in practice (which does support browser
action popups).

…evelop

* origin/develop: (31 commits)
  Performance: Delivery optimized images (#7176)
  Add `appName` message to each locale
  Remove the disk store (#7170)
  Update @hapi/subtext as per security advisory (#7172)
  Add fixes for German translations (#7168)
  Fix recipient field of approve screen (#7171)
  3box integration 2.0 (#6972)
  ci - metamaskbot - include links to dep-viz and all artifacts (#7155)
  Replace `undefined` selectedAddress with `null` (#7161)
  Add polyfill for AbortController (#7157)
  Remove redundant error logging (#7158)
  Set minimum Firefox version to v56.2 to support Waterfox (#7156)
  ci - install deps with "--har" flag to capture network activity (#7143)
  ci - create source-map-explorer build-artifacts (#7141)
  ci - build-artifacts - generate sesify-viz for inspecting deps (#7151)
  Publish GitHub release from master branch (#7136)
  fix rinkeby spelling (#7148)
  deps - move gulp-terser-js to devDeps
  test:integration - fix renamed test data file
  lint fix
  ...

* origin/master:
  Add v7.2.2 to changelog
  Update minimum Firefox verison to 56.0 (#7213)
  Version v7.2.2
  Update changelog for v7.2.1, v7.2.0, and v7.1.1
  Add `appName` message to each locale
  Version v7.2.1
  Update changelog with additional bug fixes
  Fix recipient field of approve screen (#7171)
  Replace `undefined` selectedAddress with `null` (#7161)
  Add polyfill for AbortController (#7157)
  Set minimum Firefox version to v56.2 to support Waterfox (#7156)
  Publish GitHub release from master branch (#7136)
  Update the changelog for v7.1.1 (#7145)
  build - replace gulp-uglify-es with gulp-terser-js
  Version v7.2.0
  Update changelog with 7.2.0 changes
  Allow dismissing privacy mode from popup
  Add changelog
  Version v7.1.1