Check that all our custom browser prefs are taken into account with Firefox ESR60

… both for Tor Browser and the Unsafe Browser.

• config/chroot_local-includes/usr/share/tails/tor-browser-prefs.js
• config/chroot_local-includes/usr/share/tails/chroot-browsers/common/prefs.js
• config/chroot_local-includes/usr/share/tails/chroot-browsers/unsafe-browser/prefs.js
• all calls to set_mozilla_pref
  • destination file (some are now ignored)
  • adding the user_pref optional argument may be needed in some cases

Parent Task: #15023 (closed)

Related issues

• Blocks #15334 (closed)

Original created by @intrigeri on 15703 (Redmine)