Activating menu items creates permanent space between title bar and tabs

This bug applies to all Firefox versions since Unity brought global menus. However, only Firefox is affected, I've not observed any other app with a similar issue.

What I expected:
 * Activating a menu item to simply display the relevant menu and cause no further interface changes

What happened instead:
 * Activating a menu item displays the relevant menu and introduces a space between the titlebar and tabs

The change is persistent as long as the window is open. Resizing or maximizing makes no difference. Enabling and activating menus from the title bar causes the exact same effect. Closing the window and reopening it from History's "Restore Closed Windows" removes the space. In other words, it does not affect newly opened windows.
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AddonCompatCheckDisabled: False
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
AudioDevicesInUse:
 USER PID ACCESS COMMAND
 /dev/snd/controlC0: virtouni 1649 F.... pulseaudio
BuildID: 20160425114621
Channel: Unavailable
CurrentDesktop: Unity
DistroRelease: Ubuntu 16.04
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 2016-04-26 (5 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
Locales: extensions.sqlite corrupt or missing
Package: firefox 46.0+build5-0ubuntu0.16.04.2
PackageArchitecture: amd64
PrefSources: prefs.js
ProcVersionSignature: Ubuntu 4.4.0-22.38-generic 4.4.8
Profiles: Profile0 (Default) - LastVersion=46.0/20160425114621 (In use)
RunningIncompatibleAddons: False
Tags: xenial
Themes: extensions.sqlite corrupt or missing
Uname: Linux 4.4.0-22-generic x86_64
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm cdrom dip lpadmin plugdev sambashare sudo
_MarkForUpload: True
dmi.bios.date: 11/04/2008
dmi.bios.vendor: Hewlett-Packard
dmi.bios.version: 68MDU Ver. F.0D
dmi.board.name: 30D8
dmi.board.vendor: Hewlett-Packard
dmi.board.version: KBC Version 83.0E
dmi.chassis.asset.tag: CNU73433WV
dmi.chassis.type: 10
dmi.chassis.vendor: Hewlett-Packard
dmi.modalias: dmi:bvnHewlett-Packard:bvr68MDUVer.F.0D:bd11/04/2008:svnHewlett-Packard:pnHPCompaq6720s:pvrF.0D:rvnHewlett-Packard:rn30D8:rvrKBCVersion83.0E:cvnHewlett-Packard:ct10:cvr:
dmi.product.name: HP Compaq 6720s
dmi.product.version: F.0D
dmi.sys.vendor: Hewlett-Packard