Access to IPv4-only subdomain via IPv6

[Craig McQueen]

I access the following URL, which is done via IPv6 and returns my IPv6 address:

https://craig.mcqueen.id.au/ip.php

Then I access:

https://ipv4.craig.mcqueen.id.au/ip.php

That sub-domain has only a DNS A record, which should force it to be an IPv4 access. But, somehow Firefox manages to access it via IPv6 and return my IPv6 address again.

I think Firefox should access the second URL via IPv4.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: firefox 51.0.1+build2-0ubuntu0.16.04.1
ProcVersionSignature: Ubuntu 4.4.0-57.78-generic 4.4.35
Uname: Linux 4.4.0-57-generic x86_64
AddonCompatCheckDisabled: False
ApportVersion: 2.20.1-0ubuntu2.5
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: craig 1936 F.... pulseaudio
BuildID: 20170125172221
Channel: Unavailable
CurrentDesktop: Unity
Date: Sun Feb 5 09:42:30 2017
Extensions: extensions.sqlite corrupt or missing
ForcedLayersAccel: False
IfupdownConfig:
 # interfaces(5) file used by ifup(8) and ifdown(8)
 auto lo
 iface lo inet loopback
IncompatibleExtensions: Unavailable (corrupt or non-existant compatibility.ini or extensions.sqlite)
InstallationDate: Installed on 2015-10-08 (485 days ago)
InstallationMedia: Ubuntu 15.04 "Vivid Vervet" - Release amd64 (20150422)
IpRoute:
 default via 192.168.178.1 dev eth0 proto static metric 100
 192.168.178.0/24 dev eth0 proto kernel scope link src 192.168.178.33 metric 100
Locales: extensions.sqlite corrupt or missing
MostRecentCrashID: bp-a5d28002-f6a2-4e09-9e15-4cc312161226
Plugins:
 Google Talk Plugin - /opt/google/talkplugin/libnpgoogletalk.so (google-talkplugin)
 Google Talk Plugin Video Renderer - /opt/google/talkplugin/libnpo1d.so (google-talkplugin)
 iTunes Application Detector - /usr/lib/mozilla/plugins/librhythmbox-itms-detection-plugin.so (rhythmbox-mozilla)
 Shockwave Flash - /usr/lib/flashplugin-installer/libflashplayer.so
PrefSources:
 prefs.js
 [Profile]/<email address hidden>/defaults/preferences/defaultPrefs.js
ProcEnviron:
 LANGUAGE=en_AU:en
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_AU.UTF-8
 SHELL=/bin/bash
Profiles: Profile0 (Default) - LastVersion=51.0.1/20170125172221 (In use)
RelatedPackageVersions:
 google-talkplugin 5.41.3.0-1
 rhythmbox-mozilla 3.3-1ubuntu7
RunningIncompatibleAddons: False
SourcePackage: firefox
SubmittedCrashIDs:
 bp-a5d28002-f6a2-4e09-9e15-4cc312161226
 bp-c84c5629-e2eb-4027-8054-bb86b2160724
Themes: extensions.sqlite corrupt or missing
UpgradeStatus: Upgraded to xenial on 2016-06-23 (226 days ago)
dmi.bios.date: 11/27/2008
dmi.bios.vendor: TOSHIBA
dmi.bios.version: Version 1.90
dmi.board.asset.tag: 0000000000
dmi.board.name: Portable PC
dmi.board.vendor: TOSHIBA
dmi.board.version: Version A0
dmi.chassis.asset.tag: 0000000000
dmi.chassis.type: 10
dmi.chassis.vendor: TOSHIBA
dmi.chassis.version: Version 1.0
dmi.modalias: dmi:bvnTOSHIBA:bvrVersion1.90:bd11/27/2008:svnTOSHIBA:pnTECRAP10:pvrPTSB3A-05R001:rvnTOSHIBA:rnPortablePC:rvrVersionA0:cvnTOSHIBA:ct10:cvrVersion1.0:
dmi.product.name: TECRA P10
dmi.product.version: PTSB3A-05R001
dmi.sys.vendor: TOSHIBA