Firefox homepage setting lost, changed to "about:blank" after update from 16.04 to 18.04.1

[Steve Chadsey]

After updating my system from Xubuntu 16.04 to 18.04.1 (via "do-release-upgrade"), my firefox opens a blank page instead of my home page when I launch the application.

Before:
$ egrep 'browser.startup' prefs.js-old
user_pref("browser.startup.homepage", "https://mail.google.com/mail/?shva=1#inbox");
user_pref("browser.startup.homepage_override.buildID", "20180316021607");
user_pref("browser.startup.homepage_override.mstone", "59.0.1");

After:
$ egrep 'browser.startup' prefs.js-new
user_pref("browser.startup.homepage", "about:blank");
user_pref("browser.startup.homepage_override.buildID", "20181002130007");
user_pref("browser.startup.homepage_override.mstone", "62.0.3");

To the best of my recollection I did not make this change in the Firefox preferences UI.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: firefox 62.0.3+build1-0ubuntu0.18.04.1
ProcVersionSignature: Ubuntu 4.15.0-36.39-generic 4.15.18
Uname: Linux 4.15.0-36-generic x86_64
AddonCompatCheckDisabled: False
ApportVersion: 2.20.9-0ubuntu7.4
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC1: tyr 2187 F.... pulseaudio
 /dev/snd/controlC0: tyr 2187 F.... pulseaudio
BuildID: 20181002130007
Channel: Unavailable
CurrentDesktop: XFCE
Date: Mon Oct 8 19:50:55 2018
ForcedLayersAccel: False
IfupdownConfig:
 # interfaces(5) file used by ifup(8) and ifdown(8)
 auto lo
 iface lo inet loopback
IncompatibleExtensions:
 English (South Africa) Language Pack - <email address hidden>
 English (GB) Language Pack - <email address hidden>
 Default - {972ce4c6-7e08-4474-a285-3208198ce6fd}
InstallationDate: Installed on 2016-01-05 (1007 days ago)
InstallationMedia: Xubuntu 14.04.3 LTS "Trusty Tahr" - Beta amd64 (20150805)
IpRoute:
 default via 192.168.1.254 dev eth0 proto dhcp metric 100
 192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.168 metric 100
IwConfig:
 eth0 no wireless extensions.

 lo no wireless extensions.
MostRecentCrashID: bp-0e19bace-b03b-4050-beb1-02f972150823
Plugins: Shockwave Flash - /usr/lib/flashplugin-installer/libflashplayer.so
PrefSources:
 prefs.js
 user.js
Profiles: Profile0 (Default) - LastVersion=62.0.3/20181002130007 (In use)
RfKill:

RunningIncompatibleAddons: True
SourcePackage: firefox
SubmittedCrashIDs:
 bp-0e19bace-b03b-4050-beb1-02f972150823
 bp-5a4d2dd4-5aba-4500-ab76-fac2a2150822
 bp-d1cd28b1-b27d-448b-a69c-a7d132150711
 bp-7538fe94-b454-46f4-95ed-aee302121011
 bp-cbfa4d9f-fb82-435e-af03-501332121011
UpgradeStatus: Upgraded to bionic on 2018-10-08 (0 days ago)
dmi.bios.date: 11/11/2015
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 2801
dmi.board.asset.tag: To be filled by O.E.M.
dmi.board.name: Z97-A
dmi.board.vendor: ASUSTeK COMPUTER INC.
dmi.board.version: Rev 1.xx
dmi.chassis.asset.tag: To Be Filled By O.E.M.
dmi.chassis.type: 3
dmi.chassis.vendor: To Be Filled By O.E.M.
dmi.chassis.version: To Be Filled By O.E.M.
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr2801:bd11/11/2015:svnASUS:pnAllSeries:pvrSystemVersion:rvnASUSTeKCOMPUTERINC.:rnZ97-A:rvrRev1.xx:cvnToBeFilledByO.E.M.:ct3:cvrToBeFilledByO.E.M.:
dmi.product.family: ASUS MB
dmi.product.name: All Series
dmi.product.version: System Version
dmi.sys.vendor: ASUS