firefox default search engine

In firefox 62, if I change the default search engine in preferences, it comes back to google every time I restart the browser. I 've also tried with an external qwant plugin that sets qwant as default but it always comes back to google.
So there is no way for me to change my default engine unless I specify it on every startup.

It seems that we are numerous to have the problem. It seems that we are all on ubuntu.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: firefox 62.0+build2-0ubuntu0.18.04.3
ProcVersionSignature: Ubuntu 4.15.0-34.37-generic 4.15.18
Uname: Linux 4.15.0-34-generic x86_64
AddonCompatCheckDisabled: False
ApportVersion: 2.20.9-0ubuntu7.3
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: eric 4357 F.... pulseaudio
BuildID: 20180905220717
Channel: Unavailable
CurrentDesktop: Unity:Unity7:ubuntu
Date: Wed Sep 12 09:17:30 2018
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 2016-10-21 (690 days ago)
InstallationMedia: Ubuntu 16.10 "Yakkety Yak" - Release amd64 (20161012.2)
IpRoute:
 default via 192.168.0.254 dev wlp2s0 proto dhcp metric 600
 169.254.0.0/16 dev wlp2s0 scope link metric 1000
 192.168.0.0/24 dev wlp2s0 proto kernel scope link src 192.168.0.112 metric 600
Locales: extensions.sqlite corrupt or missing
MostRecentCrashID: bp-cfea9b8a-335d-4bc5-8b0c-d3a640171005
PrefSources: prefs.js
Profiles: Profile0 (Default) - LastVersion=62.0/20180905220717 (In use)
RunningIncompatibleAddons: False
SourcePackage: firefox
SubmittedCrashIDs:
 bp-cfea9b8a-335d-4bc5-8b0c-d3a640171005
 bp-a4b17215-76a6-49de-a4aa-0b65e0170910
Themes: extensions.sqlite corrupt or missing
UpgradeStatus: Upgraded to bionic on 2018-05-25 (109 days ago)
dmi.bios.date: 07/18/2018
dmi.bios.vendor: Dell Inc.
dmi.bios.version: 1.8.0
dmi.board.name: 0N8J4R
dmi.board.vendor: Dell Inc.
dmi.board.version: A00
dmi.chassis.type: 9
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvr1.8.0:bd07/18/2018:svnDellInc.:pnPrecision5510:pvr:rvnDellInc.:rn0N8J4R:rvrA00:cvnDellInc.:ct9:cvr:
dmi.product.family: To be filled by O.E.M.
dmi.product.name: Precision 5510
dmi.sys.vendor: Dell Inc.