Context menus spawn in a way that causes unintended clicks when using Firefox CSD

[Douglas Silva]

HOW TO REPRODUCE
1 - First, open Firefox, enable Client Side Decoration, restart the program.
2 - Open some webpage, make sure you navigate a bit, leaving behind pages that can be returned to.
3 - Then right click anywhere on the current page. On my system, the context menu 'back' button appears right beneath the mouse cursor, and a click is automatically inserted there, making the page 'go back' everytime you right click with the mouse.

Running Xubuntu 18.04.1 LTS
Firefox 64.0

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: firefox 64.0+build3-0ubuntu0.18.04.1
ProcVersionSignature: Ubuntu 4.15.0-42.45-generic 4.15.18
Uname: Linux 4.15.0-42-generic x86_64
AddonCompatCheckDisabled: False
ApportVersion: 2.20.9-0ubuntu7.5
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC1: dsilva 1420 F.... pulseaudio
 /dev/snd/controlC0: dsilva 1420 F.... pulseaudio
BuildID: 20181207224003
Channel: Unavailable
CurrentDesktop: XFCE
Date: Thu Dec 13 12:35:17 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 2018-12-13 (0 days ago)
InstallationMedia: Xubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
IpRoute:
 default via 10.0.0.1 dev wlp6s0 proto dhcp metric 600
 10.0.0.0/24 dev wlp6s0 proto kernel scope link src 10.0.0.102 metric 600
 169.254.0.0/16 dev wlp6s0 scope link metric 1000
Locales: extensions.sqlite corrupt or missing
PrefSources: prefs.js
Profiles: Profile0 (Default) - LastVersion=64.0/20181207224003
RunningIncompatibleAddons: False
SourcePackage: firefox
Themes: extensions.sqlite corrupt or missing
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 09/27/2018
dmi.bios.vendor: Dell Inc.
dmi.bios.version: A16
dmi.board.name: 09WGNT
dmi.board.vendor: Dell Inc.
dmi.board.version: A00
dmi.chassis.type: 9
dmi.chassis.vendor: Dell Inc.
dmi.modalias: dmi:bvnDellInc.:bvrA16:bd09/27/2018:svnDellInc.:pnInspiron5458:pvr:rvnDellInc.:rn09WGNT:rvrA00:cvnDellInc.:ct9:cvr:
dmi.product.name: Inspiron 5458
dmi.sys.vendor: Dell Inc.

[Theo Linkspfeifer (lastonestanding)]

As possible workaround try setting ui.context_menus.after_mouseup to 'true' in about:config.